“Attn” means “attention” and marks the attention line on a letter. The attention line tells a business or organisation who should get a letter or package.
When you send a letter or package to a large organisation, you use “Attn” or the attention line. When the letter or package gets to the mail room of an organisation, the attention line makes it clear who it is for.
The attention line is useful for people who want to talk to a person in their role as a representative of an organisation instead of as a person. On a letter or package, the attention line should go above the name of the organisation.
The United States Postal Service tells people not to use “attn” when addressing a package. Instead, they should write the name of the specific recipient above the name of the organisation. In a business letter, the line that says “Attention” goes above the greeting.
“Attn” is just slang for the word “attention” when it is not in a letter. The abbreviation is often used on the internet by people who are not professionals or in works of literature that are meant to break the rules of grammar.
Attention is defined as focusing on one thing and ignoring everything else around you. This definition is a good match for how the abbreviation is used to address a piece of mail.