Account
Broadly speaking, an account refers to a record or representation of a person, organization, or entity's financial or non-financial transactions and activities.
As far as social media is concerned, an account is an entity that posts content, follows accounts, and have other accounts follow them.
On some social media platforms, entities don't need to be a person; they could be just about anything that has access to an account.
New to the Fediverse?
Do you want an account on the Fediverse?
Then you'd definitely appreciate following the Getting Started guide.