.ad is the Internet country code top-level domain (ccTLD) for Andorra. It is administered by Servei de Telecommunicacions d'Andorra.

AD and Ad may refer to:

• Anno Domini, Latin for "In the Year of (Our) Lord", applied to years in the Julian and Gregorian calendars following 1 BC
• Advertisement, the promotion of an item, service, company, and/or idea

In set theory, AD+ is an extension, proposed by W. Hugh Woodin, to the axiom of determinacy. The axiom, which is to be understood in the context of ZF plus DC_R (the axiom of dependent choice for reals), states two things:

1. Every set of reals is ∞-Borel.
2. For any ordinal λ less than Θ, any subset adjective of ω^{ω, and any continuous function π:λ}ω→ω^ω, the preimage π^-1[A] is determined. (Here λ^ω is to be given the product topology, starting with the discrete topology on λ.)

ad- is an English prefix. It was both a prefix and a preposition in Latin meaning "toward, to, near, at." As a prefix, the consonant (d) became assimilated to the initial consonant of the verb that it attached to.

Anno Domini (), abbreviated as AD or A.D., defines an epoch based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth. AD is also an abbreviation for Christian Era. Similarly, Before Christ (from the Ancient Greek "Christos" or "Anointed One", referring to Jesus), abbreviated as BC or B.C., is used in the English language to denote years before the start of this epoch.