Angel
Angels are spiritual beings believed to serve various deities. Although their name comes from the Latin for "Messenger", their association with the Abrahamic religions in the real world means that the standard perception of angels is as magical spirit-beings allied to Good, who stand in opposition to fiends such as demons and devils. Actual use of the term "Angel" can be seen as rather taboo due to various unpleasant attitudes and views towards real-life religions, so sometimes Celestial is preferred as a less-loaded moniker.
In Dungeons & Dragons
In Dungeons & Dragons, "Angel" could traditionally be seen as a generic term for an Outsider from the Upper Planes, although traditionally the term Celestial is preferred for that, due to the Satanic Panic of the 80's.
There are four distinct species of Celestial in D&D; the Lawful Good Archons, the Neutral Good Guardinals, the Chaotic Good Eladrin, and the Any Good Aasimon, who are literally the "conventionally angelic Celestials".
In 4th edition, Celestials as a whole disappeared and were replaced with Angels. Harkening all the way back to their original Biblical portrayl, the Angels of 4th edition are an Unaligned race of Immortals; these entities rise from the Astral Sea and exist to serve all gods, good and evil, with equal loyalty and dispassionate competence. Baatezu in this edition became "Fallen Angels", cursed for betraying & murdering their god. Some angels become fascinated with mortal life and choose to forsake most of their immortality to live closer to them, becoming Devas.
In 5th edition, "angel" is the name given to the Aasimon, who no longer try to hide what they're based on.
In Pathfinder
Pathfinder's Angels are literally just AD&D's Aasimon without trying to obfuscate what they're based on.