A leading actor, leading actress, star, or simply lead, plays the role of the protagonist of a film or play. The word lead may also refer to the largest role in the piece and leading actor may refer to a person who typically plays such parts or an actor with a respected body of work. Some actors are typecast as leads, but most play the lead in some performances and supporting or character roles in others.
Sometimes there is more than one significant leading role in a dramatic piece, and the actors are said to play co-leads; a large supporting role may be considered a secondary lead. Award nominations for acting often reflect such ambiguities.
Thus, sometimes two actors in the same performance piece are nominated for Best Actor or Best Actress -- categories traditionally reserved for leads. For example, in 1935 Clark Gable, Charles Laughton and Franchot Tone were each nominated for the Best Actor Academy Award for Mutiny on the Bounty.
There can even be controversy over whether a particular performance should be nominated in the Best Actor/Actress or Best Supporting Actor/Actress category.
A title role is often but not necessarily the lead.