The Tax Cuts and Jobs Act of 2017 repealed the shared responsibility payment, which is a penalty for not having minimum essential health coverage (and not being exempt from this requirement). But the repeal doesn’t impact 2018; it only provides relief after 2018.
Gross income less allowable adjustments, such as IRA, alimony, and Keogh deductions. AGI determines whether various tax benefits are phased out, such as personal exemptions, itemized deductions, and the rental loss allowance and modified adjusted gross income (MAGI).