What does "costar" mean?

A person who shares star billing with another.

What is a synonym for "costar"?

Common synonyms include: act together, co-lead, co-lead actor, co-leading actor, co-star, coact.

Pronounced //ˈkəʊstɑː(ɹ)//.

/ˈkəʊstɑː(ɹ)/ noun, verb

noun, verb ·Rare ·Advanced level

Noun

1
A person who shares star billing with another.
"The once famous actor objected to his costar having a bigger dressing room."
2
A person who slightly lacks the status to be considered a star.
"Alas, always a costar but never a star."

Verb

1
To perform with the billing of a costar. intransitive
"People thought his career was over but now he will get to costar on Broadway next month."
2
To feature (a number of star performers) jointly. transitive
"Purchasing a snowplow transforms Homer into a new man. Mr. Burns' laziest employee suddenly becomes an ambitious self-starter who buys ad time on local television at 3:17 A.M^([sic]) (prime viewing hours, Homer gingerly volunteers, for everyone from alcoholics to the unemployable to garden-variety angry loners) and makes a homemade commercial costarring his family."

"The once famous actor objected to his costar having a bigger dressing room."

From co- + star.