Modelling the entire instantaneous forward-rate curve f(t,T) directly under no-arbitrage. Under mathbbQ, df(t,T) = α(t,T),dt + σ(t,T),dW_t, and the HJM drift condition forces α(t,T) = σ(t,T)int_t^T σ(t,u),du — the drift is uniquely determined by the volatility, so calibration is purely about specifying σ(·,·). The flip side is that f(t,T) is generally non-Markovian (infinite-dimensional state), making numerical pricing of path-dependent products expensive; LMM is the discrete-tenor cousin that restores tractability.