Significant other and Somebody

Significant other noun - A person (not necessarily a spouse) with whom you cohabit and share a long-term sexual relationship.

Somebody and significant other are semantically related. In some cases you can use "Somebody" instead a noun phrase "Significant other".

Somebody noun - A person who is widely known and usually much talked about.

Significant other and somebody are semantically related. Sometimes you can replace phrase "Significant other" with "Somebody", this terms are similar.