An Alias is an alternate email address linked to your existing account. Emails sent to your alias arrive in the same inbox, and you can also send emails from it without switching accounts.
How do I add an Alias?
Go to Settings | Accounts | [select the account] | Aliases | Edit Aliases | +, then enter the alias email address and a display name.
Note: The alias must be set up with your email provider first. Here are a few guides for common providers:
How do I use an Alias?
When composing an email, tap your name and address at the top of the screen and select your alias.
The same Aliases screen also has Default From (Aliases), which lists the account's addresses so you can pick its default From address. A check mark shows the current choice.