theirs

We don’t traditionally think of theirs as having synonyms, but there are several situations in which it can replace—or be replaced with—another term.

their

Sometimes you can reword what you’re saying to use their instead of theirs. For example, instead of saying This seat is theirs, you could say This is their seat. 

their own 

The phrase their own is sometimes used as another way of saying theirs. For example, instead of saying After tasks are assigned, each group should focus only on theirs, you could say After tasks are assigned, each group should focus only on their own.

Other possessives

Instead of saying things like This house is his and hers, we usually use the plural form theirs, as in This house is theirs. Sometimes, though, it makes more sense to separate who owns what by using separate possessive words. For example, instead of saying These vehicles are theirs, you could say This car is his and this truck is hers.

Strictly speaking, theirs doesn’t have a definitive antonym in the same sense that short is the antonym of long. Of course, possession and association are often distinguished by using contrasting possessives, such as in statements like This responsibility is theirs—it’s not ours. In this sense, all of the other possessive words could be considered the opposite of theirs, including mine, hers, his, and ours.

On the other hand, one person’s possession of something does not mean that someone else can’t possess it as well. This is indicated in expressions like what’s mine is theirs.

Theirs is a pronoun. It’s a form of the possessive case of the personal pronoun they that’s used as a predicate adjective (a type of adjective that modifies or describes the subject of a sentence or clause and is linked to the subject by a linking verb, such as a form of the verb be), as in Those books are theirs. It can also be used as a pronoun that functions as the subject, as in Theirs are the red ones. 

Like the similar possessive words mine, yours, hers, and ours, theirs stands alone and typically never accompanies another noun as a modifier.

In contrast, the word his can be used as both a possessive pronoun (as in That jacket is his) or a possessive adjective (as in That is his jacket).

Theirs is used along with third person pronouns (like they and them), which refer to anyone other than the person doing the addressing or the ones being addressed. Other third person possessive forms include their, his, her, and hers. In contrast, first person pronouns (like I, me, we, and us) refer to the speaker or writer themselves. First person possessive forms include my, mine, our, and ours. Second person pronouns (like you and yourself) are used to address the person being spoken to or reading. Possessive forms of these include your and yours.

Theirs can be both plural and singular. They has a long history of being used to refer to an unspecified individual or to a person whose gender and other personal details are unknown or irrelevant, and theirs can also be used as a singular in this way, as in I’m not sure who that is, but I think this car is theirs. Sentences like this sometimes use constructions like his or hers, but the use of theirs has become very common in such instances, with many people preferring theirs as less awkward and more inclusive.

A use of theirs that has become more widespread is as a singular possessive form for those who use the singular, gender-inclusive personal pronoun they due to identifying as nonbinary or having a gender identity that exists between or beyond the spectrum of strictly male or female. Grammatically, theirs is used in the same way as the gender-specific terms like his and hers, but it’s gender-neutral.