Traceability has been defined by Gotel et al, as "the ability to follow the life of a requirement in both a forward and backward direction"[1].
Jarke et al identified four types of traceability that they referred to as (i) backwards from requirements, (ii) forwards to requirements, (iii) backwards to requirements, and (iv) forward from requirements [2].
1. Orlena Gotel, Anthony Finkelstein: Contribution structures (Requirements artifacts). RE 1995: 100-107.
2. Matthias Jarke: Requirements Tracing - Introduction. Commun. ACM 41(12): 32-36 (1998)