The Ultimate is the fourth ability of most Champions. The ultimate is considered a champion's defining ability and is oftentimes considered their strongest. Ultimates vary greatly between champions, with some having passive effects ( 贾克斯的 Grandmaster's Might), some having global effects ( Karthus' Requiem), some being spammable from having low cooldowns ( Kassadin's Riftwalk), some being auras ( Sivir's On The Hunt) and some summoning other units ( Annie's Summon Tibbers). Whether an ultimate is considered a champion's strongest ability or not, using your ultimate effectively is crucial to playing any champion. Ultimates are defined by being able to be taken at levels 6, 11, and 16.
It is also possible to tell whether or not an allied Champion's ultimate is ready or not, by the indicator. The indicator is the green dot which is placed right beside the respective champion's portrait on the left side of your screen. When the green light is on, the champion's ultimate is ready. If the light is off, the champion has either not learned the abilty or it is currently on cooldown.
There is currently only one champion that doesn't have an Ultimate: Udyr. He has four Stances that are all trainable up to level 5. Karma's ultimate is also special, being available from level 1 and not requiring any skill points; instead, all her other abilities can be leveled up to 6. 杰斯的Mercury Hammer Mercury Hammer and Mercury Cannon Mercury Cannon and 伊莉丝的Spider Form Spider Form and Human Form Human Form are available at level 1, but can still be leveled up at level 6, 11 and 16, making them special in having 4 ranks.