A cis geometric isomer, i

A cis geometric isomer, i.e. a type of diastereomer found in alkenes.

Maleic acid is otherwise known as cis-butenedioic acid, which has a double bond that is flanked by two carboxylic acid groups. Maleic acid, a geometric isomer, is thus a diastereomer and therefore a stereoisomer.

An important distinction to make here is that maleic acid is the cis isomer, whereas fumaric acid is the trans isomer!

If someone asks you to draw maleic acid, and you draw trans-butenedioic acid, you haven't drawn maleic acid! You've drawn its trans, geometric isomer, fumaric acid.