civis

Civis, civis is third declension and of common gender: the same form serves a man and a woman, and only the adjective tells you which. It is an i-stem, so the genitive plural is civium and the accusative plural can be cives or the older civis.

Civis Romanus sum was a legal claim with teeth, not a boast. It bought you a trial, protected you from scourging and the cross, and that is exactly what Paul is doing with it in Acts. Note too that civis very often means "fellow citizen": cives nostri is "our own countrymen", and the pointed opposite of civis is hostis, the outsider. The body they collectively make up is the civitas.