Financial Glossary
Plain-English definitions of the terms used throughout this site — and across personal finance in general.
Compounding & Interest
Compound Interest
Interest earned on both the original principal and on the accumulated interest from prior periods. In other words, interest that earns interest. The longer the time horizon, the more dramatic the effect. See the complete guide to compound interest.
Simple Interest
Interest calculated only on the original principal, never on previously-earned interest. Simple interest grows linearly; compound interest grows exponentially. Simple interest is common in short-term loans; compound interest dominates long-term investing.
Principal
The original amount of money deposited, invested, or borrowed — before any interest is added. In a $10,000 deposit that grows to $12,000, the principal is $10,000 and the interest is $2,000.
Compounding Frequency
How often interest is calculated and added to the balance: annually, semi-annually, quarterly, monthly, daily, or continuously. More frequent compounding produces slightly higher effective yields, but the difference is smaller than most people expect.
APR (Annual Percentage Rate)
The stated yearly interest rate, not accounting for compounding within the year. APR is typically used for loans (credit cards, mortgages) and represents the nominal rate.
APY (Annual Percentage Yield)
The effective yearly rate including the effect of compounding. APY is typically used for savings accounts and CDs. A 5% APR compounded monthly has an APY of about 5.12%.
Rule of 72
A quick mental shortcut: divide 72 by the annual return rate to estimate how many years it takes to double your money. At 8% annual return, your money doubles roughly every 9 years (72 ÷ 8). Accurate enough for rough mental math.
Time Value of Money
The principle that a dollar today is worth more than a dollar in the future, because today's dollar can be invested and earn returns. This is the mathematical foundation behind why starting early matters so much.
Retirement & FIRE
FIRE (Financial Independence, Retire Early)
A movement and lifestyle focused on aggressive saving and investing — typically 50% or more of income — to reach financial independence decades earlier than traditional retirement. Once your investments can support your expenses indefinitely, work becomes optional.
FIRE Number
The portfolio balance at which your investment returns can sustainably cover your annual expenses. Commonly calculated as 25 times your annual expenses (based on a 4% withdrawal rate). If you spend $50,000 per year, your FIRE number is $1,250,000.
4% Rule
A retirement guideline suggesting you can withdraw 4% of your initial portfolio each year (adjusted for inflation) with a high probability of not running out of money over a 30-year retirement. Based on the Trinity Study of historical market data. It's a starting point, not a guarantee.
Withdrawal Rate
The percentage of a retirement portfolio withdrawn each year to cover expenses. Lower withdrawal rates (3–3.5%) are safer for long retirements; higher rates (5%+) carry more risk of depletion. The 4% rule is the most cited baseline.
Coast FIRE
The point where your existing investments will grow through compound interest alone to reach your FIRE number by retirement age — no additional contributions needed. After hitting Coast FIRE, you only need to cover current expenses; your retirement is already "baked in." See the Coast FIRE calculator.
Barista FIRE
A semi-retirement variant where you have enough saved that a low-stress part-time job (the archetypal "barista" job, often taken for health insurance) covers your remaining expenses while your portfolio continues to grow untouched.
Lean FIRE
A minimalist version of FIRE aimed at a modest lifestyle, typically requiring $500K–$1M in invested assets. Lean FIRE prioritizes reaching financial independence quickly by keeping expenses very low.
Fat FIRE
The opposite of Lean FIRE — reaching financial independence with enough wealth to support a comfortable or even luxurious lifestyle. Usually requires $2.5M or more, depending on spending.
Savings Rate
The percentage of your take-home (or gross) income that you save and invest. A higher savings rate is the single biggest lever for reaching FIRE quickly. A 50% savings rate can enable retirement in roughly 17 years, while a 10% rate takes over 50 years.
Sequence of Returns Risk
The risk that poor investment returns early in retirement — when you're simultaneously withdrawing — can permanently damage your portfolio, even if average long-term returns are fine. Order matters once you start withdrawing; it doesn't during pure accumulation.
Accounts & Investment Vehicles
401(k)
An employer-sponsored retirement account in the U.S. that lets you contribute pre-tax dollars (reducing current taxable income). Many employers match a percentage of contributions. Withdrawals in retirement are taxed as income. Contribution limits are set annually by the IRS.
Roth IRA
An individual retirement account funded with after-tax dollars. Contributions aren't deductible, but all qualified withdrawals in retirement — including decades of investment growth — are completely tax-free. Extremely powerful for long time horizons.
Traditional IRA
An individual retirement account that may offer tax-deductible contributions (depending on income and workplace plan coverage). Growth is tax-deferred, and withdrawals in retirement are taxed as ordinary income.
Index Fund
A mutual fund or ETF designed to track a market index (like the S&P 500) rather than trying to beat it. Index funds typically have very low fees and have historically outperformed the majority of actively-managed funds over long periods.
ETF (Exchange-Traded Fund)
A basket of securities that trades on stock exchanges like an individual stock. Most ETFs track an index. They typically have low expense ratios and more tax efficiency than traditional mutual funds.
HYSA (High-Yield Savings Account)
A savings account — usually at an online bank — that pays meaningfully higher interest than a typical brick-and-mortar bank account. Federally insured and highly liquid. Commonly used for emergency funds.
CD (Certificate of Deposit)
A time-deposit account with a fixed interest rate and fixed term (3 months to 5+ years). Usually pays more than savings accounts, but early withdrawal incurs a penalty. Predictable, low-risk, federally insured.
Returns & Risk
Nominal Return
The raw rate of return on an investment, not adjusted for inflation. A portfolio that grew 8% in a year with 3% inflation had an 8% nominal return but only a 5% real return.
Real Return
The rate of return after subtracting inflation. Real returns matter far more than nominal returns for long-horizon planning, because they reflect actual increases in purchasing power. Long-run real stock market returns are roughly 6–7%.
Inflation
The general rise in prices over time — and the corresponding fall in the purchasing power of each dollar. In the U.S., inflation has historically averaged about 3% per year, though it varies significantly year to year.
CAGR (Compound Annual Growth Rate)
The constant annual rate that would produce the same total growth as an investment's actual (uneven) returns. CAGR smooths out year-to-year volatility and makes long-term comparisons easier.
Expense Ratio
The annual fee that funds charge as a percentage of assets. A 1% expense ratio on a $100,000 balance costs $1,000 per year. Over decades, high expense ratios dramatically erode compound growth — which is why low-cost index funds are so widely recommended.
Dollar-Cost Averaging (DCA)
Investing a fixed amount on a regular schedule, regardless of market price. Removes the temptation to time the market and smooths out your average purchase price. Automatic 401(k) contributions are a form of DCA.
Asset Allocation
The mix of asset classes (stocks, bonds, cash, real estate) in a portfolio. Allocation is the primary driver of long-term returns and risk — more so than picking individual investments. Common allocations shift more toward bonds as retirement approaches.
Diversification
Spreading investments across many different assets, sectors, and geographies to reduce the impact of any single one performing poorly. "Don't put all your eggs in one basket." A broad index fund provides instant diversification across hundreds or thousands of companies.
Net Worth
Total assets minus total liabilities. Assets include cash, investments, home equity, and personal property; liabilities include mortgages, student loans, credit card debt, and other obligations. Net worth is the single clearest measure of financial health over time.
Liquidity
How quickly an asset can be converted to cash without meaningful loss of value. Savings accounts are highly liquid; real estate and retirement accounts (before age 59½) are much less so. Liquidity matters for emergency funds and short-term needs.