Sexual act refers to any act of sexual intercourse. It also means penetration however slight by any object into the genital or anal opening of another person's body. Definitions are primarily governed by state criminal laws, which vary by state.
Example of a State Statute defining Sexual Act
In North Carolina, Gen. Stat. § 14-27.1 defines sexual act as follows:
N.C. Gen. Stat. § 14-27.1
[Chapter 14. Criminal law ;Subchapter 03 .Offenses against the person; Article 7a. Rape and other sex offenses]
§ 14-27.1. Definitions
As used in this Article, unless the context requires otherwise:
(4) "Sexual act" means cunnilingus, fellatio, analingus, or anal intercourse, but does not include vaginal intercourse. Sexual act also means the penetration, however slight, by any object into the genital or anal opening of another person's body: provided, that it shall be an affirmative defense that the penetration was for accepted medical purposes.