Colloquial abbreviation of 'mail address' (email address); used in casual speech to refer to an email address.
連絡先を交換するとき、メアドを教えてください。
When exchanging contact information, please tell me your email address.
登録フォームにメアドを入力してください。
Please enter your email address in the registration form.
メアドを間違えて送ってしまったので、再送します。
I sent it to the wrong email address, so I'll resend it.