[MrPedantic]

Hello Emily

I would call it the "close" or the "ending". There may be another phrase, though – if so, maybe another member will know.

Here are some options:

Yours sincerely - for use in letters and emails, when you start the letter "Dear Mr X".

Yours faithfully - for use in letters and emails, when you start the letter "Dear sir".

Best wishes - for use in a greetings card; increasing used in letters and emails too.

Best regards - for use in a business email.

Kind regards - for use in a business email.

Cheers - for use in an email to a friend.

See you later - for use in an email to a close friend.

Other members may be able to think of other phrases!

Bye
MrP