straight-line simulatability
Here, "straight-line" is contrasted with "rewinding". A simulator is "straight-line" if it does not "rewind" the party it is doing the simulation for.

For instance, in a zero-knowledge protocol, the simulator usually rewinds the "verifier". In the "straight-line" sense, this rewinding does not happen.

I first saw the term "straight-line simulator" in Rafael Pass's paper (On Deniabililty in the Common Reference String and Random Oracle Models. (CRYPTO'03)) and M.Sc. thesis (Alternative Variants of Zero-Knowledge Proofs).

Edit: I found an earlier paper: Concurrent Zero-Knowledge: Reducing the Need for Timing Constraints by Cynthia Dwork and Amit Sahai, which dates back to 1998. For more pointers, see Alon Rosen's comment below.