1. The act of posting a sexy or alluring photo of oneself on social media in an attempt to elicit likes or attention, often from one person in particular (such as an ex or a crush). A: "Whoa, look at all those likes!" B: "I'm not gonna lie—that pic was a thirst trap so that my ex can see what he's missing out on."
2. Someone who one considers very attractive, usually on social media. Did you see that shirtless pic Ryan posted today? Man, he's a real thirst trap.