Colloquial abbreviation of メールアドレス. Very common in casual conversation and texting.
See also: メールアドレス
メアド教えてくれる？
Can you give me your email address?
I gave him my email address.
新しいメアドに変更しました。
I changed to a new email address.
新しい友達とメアドを交換した。
I exchanged email addresses with my new friend.
I registered my email address on the site.