A good rule of thumb might be, "A support ability is one that puts a buff icon on another player." This cuts out loose definition stuff like, "Rampart is a support ability because it helps the healer spend less MP." or "Blood for blood is a support ability because it helps prevent the tank from dying from a mob beating on him for too long."

WHM: Protect, Stoneskin, Regen, Medica II
SCH: Eye for an Eye, Adloquiem, Succor, Sacred Soil, Whispering Dawn, Fey Covenant, Fey Illumination, Fey Glow, Fey Light
SMN: Eye for an Eye
BLM: Apocatastasis
BRD: Mage's Ballad, Army's Paeon, Swiftsong
MNK: Mantra
NIN: Goad
PLD: Cover

This definition, though, doesn't include some abilities that are pretty clearly support, such as:

SCH: Physick, Leeches, Embrace, Lustrate
WHM: Esuna, Cure, Cure II, Cure III, Medica

It also doesn't include abilities that debuff the mob in such a way that it makes your party capable of dealing more damage, such as:

BRD: Foe Requiem
MNK: Dragon Kick
DRG: Disembowel
WAR: Storm's Eye