One phrase, many different charges
In everyday speech, sexual assault can mean anything from unwanted touching over clothing to rape. New York does not use it as the name of a single crime; conduct of this kind is charged under several offense names, rape and sexual abuse among them, and most are divided into degrees. Other states and federal law organize the same ground differently. Which name applies depends on the specific conduct alleged and the circumstances around it, including questions of age and consent. That is why the exact allegation matters far more than the label someone used in a conversation or a text.
If you were the one harmed
You do not have to decide everything at once. Medical care comes first, and in many places evidence can be collected at a hospital and held for a period without an immediate decision about going to the police. Keep messages, call logs, and the names of anyone you spoke with afterward, and avoid deleting anything even when it is painful to read. Advocacy organizations and rape crisis programs can walk you through reporting options without pressure. A criminal case is brought by the prosecutor, while a civil claim is a separate path that belongs to you, and the two can move on different timelines.
If you have been accused
The urge to explain is strong, and acting on it is usually a mistake. A call from a detective, a message from a school, or a note from human resources is an invitation to make statements that will be kept and examined later. Do not contact the person who made the complaint, directly or through friends, and do not post about it. Keep your phone, accounts, and messages as they are, since deleting anything can look worse than whatever was deleted. In a first conversation we work out which processes are open, who is likely to be contacted, and what you should say to whom while the facts are still being gathered.