Social Security serves as a key source of income for countless retirees and disabled individuals. It's also an extremely complex program loaded with rules and terminology. If you're attempting to learn about Social Security (which is something you should do, regardless of how old you happen to be), here are a few key terms you'll need to understand.

1. OASDI

OASDI stands for old age, survivors, and disability insurance, and in the context of your paycheck, it's the tax used to fund the Social Security program. The current OASDI tax rate is 12.4%. If you work for an outside company, you'll lose half that amount of your earnings up to a certain income limit, while your employer will pay the remaining 6.2%. If you're self-employed, however, you'll pay the full 12.4% up front.

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2. SSI

SSI stands for supplemental security income, and it's different from OASDI in that it's a program funded by general tax revenues, not Social Security taxes. SSI is designed to help those who are over 65, blind, or disabled with limited financial resources keep up with their basic needs.

3. FICA Tax

FICA stands for the Federal Insurance Contributions Act. It's the tax that's withheld from your salary or self-employment income that funds both Social Security and Medicare. For the current year, FICA tax equals 15.3% of earned income up to $127,200 (12.4% for Social Security and 2.9% for Medicare), but those making above $127,200 will continue to pay 2.9% FICA tax on income exceeding that threshold. In 2018, the earnings cap will rise to $128,700.

4. Social Security credits

In order to collect Social Security benefits, you must earn enough credits during your working years. In 2017, you'll receive one credit for every $1,300 in earnings, up to a maximum of four credits per year. For 2018, the value of a single credit will rise to $1,320 of earnings. Those born in 1929 or later need 40 credits to qualify for benefits in retirement.

5. AIME

AIME stands for average indexed monthly earnings, and it's used to calculate your personal Social Security benefit. The amount you receive from Social Security is based on your highest 35 years of earnings. To arrive at your AIME, your past earnings are adjusted for inflation so that they don't lose value.

6. Full retirement age

Your full retirement age, or FRA, is the age at which you're eligible to collect your Social Security benefits in full. FRA is based on your year of birth, and for today's older workers, it's 66, 67, or 66 and a number of months. Though you're allowed to claim benefits prior to reaching FRA (the earliest age is 62), doing so will cause you to collect a reduced benefit amount -- permanently.