In the early episodes of Smallville, none of the main case are aware of Clark's Kryptonian past and so everyone refers to Kryptonite simply as "meteor rock".
But at some point, cast members in the know switch to using the word Kryptonite instead. For example, in episode 507 "Splinter" Martha, Jonathan, and Chloe all refer to meteor rocks as kryptonite.
Does anyone know when the term was introduced in the show and under what circumstances?
Based on the link posted by BCdotWEB: the first instance of the word Krytonite is in episode 218 "Visitor"—several episodes after Clark meets Virgil Swan, who informs him about his Kryptonian origins.
A transcript of the episode reveals the relevant dialogue (which occurs at around 26:30 on the DVD):
Pete: Well, if you really want to know [whether Cyrus is Kryptonian or not], I could go up to him with a piece of meteor rock.
Clark: No, it's called Kryptonite.
Pete: Kryptonite. Is that with a C or a K?
At this point in time, the following characters know about Clark's Kryptonian identity: Clark, Virgil Swan, Pete, Jonathan, Martha, and Jor-El (and perhaps some of Swan's staff), any of whom could have been the first to use the term off-screen. His response in the above quote suggests that the term was not coined by Pete, and it seems most likely that Clark was the first to use it.
