This is how the IRS defines AGI: Definition of Adjusted Gross Income | Internal Revenue Service (irs.gov)
If you need to find your last year's AGI, please see the below TurboTax help article:
It is your adjusted gross income. It can be found on your previous year income tax form.
AGI is the abbreviation for Adjusted Gross Income. That is your TOTAL income plus/minus ADJUSTMENTS to your income. Some examples of adjustments to gross income: jury duty pay (increase), health savings account contributions (decrease), educator expenses (decrease), student interest expense (decrease), penalty on early withdrawing of IRA (increase), retirement contributions (decrease).
The reason why Adjusted Gross Income (AGI) is important is because a lot of limits are defined in terms of your AGI; e.g., if your AGI is too high, you can't deduct IRA contributions, etc. Of course, this is the IRS, so nothing is simple. Limits are often expressed as an MAGI (Modified Adjusted Gross Income), which is your AGI plus or minus a few more things. The MAGI is differently slightly differently for every category that requires it (that is, there is no one single MAGI).