被告人 is the formal legal term used in criminal cases. In civil cases, the term is simply 被告 (bèigào). 原告 (yuángào) means plaintiff.
grammar
被 here functions as the passive marker, making 告 (to accuse) into 'the one who is accused.' This 被 + verb + 人 pattern appears in many legal terms: 被害人 (victim), 被保险人 (the insured).