A fake or stolen online identity created or used for the purposes of beginning a deceptive relationship.

you think you're talking to a fit bird and your actually talking to a 40-year-old obese plumber called Steve from Dorset who is wanking into a half-full bag of wotsits.

Catfish is someone who makes a fake account of someone on social media (Instagram, Facebook, tinder) to lure into a relationship with a false identity.

A catfish is someone who pretends to be someone they're not using Facebook or other social media to create false identities, particularly to pursue deceptive online romances.

Someone who pretends to be someone they're not online to create false identities, particularly to pursue deceptive online romances.

someone who pretends to be someone they are not online to create false identities, particularly to pursue deceptive online romances.

From the 2010 movie Catfish

Despite what the other definitions on here say, a person who catfishes online is not a catfish. The term "catfishing" (online) derives from real-world fishing. The person who is catfishing puts the bait out there, and waits for someone to bite. The person who falls for the bait and believes the catfisher is the catfish.