In Dota 2, a hero's most powerful ability is commonly called its ultimate.
An ultimate is generally the defining or most powerful ability in a hero’s kit. In the traditional progression system, heroes unlock it at level 6, later than their basic abilities, and it often has a long cooldown or a substantial mana cost. The term is frequently shortened to “ulti.”
Ultimates vary enormously. Some deal heavy area damage, while others stun enemies, protect allies, summon units, alter a hero’s form, or create powerful strategic effects. A strong ultimate is not always a damaging spell; its value depends on timing, positioning, and the situation.
Ultimate is sometimes confused with a passive, talent, or ability upgrade. A passive works without being actively cast, a talent modifies a hero’s options, and a rune is a map pickup. None of those is the standard name for a hero’s signature top-tier ability.