1. delay in action of the sensing element of a control device due to the time required for the sensing element to reach equilibrium with the variable being controlled, i.e., temperature lag, flow lag, etc.

2. retardation of an output with respect to the causal input.

3. delay attributable to the transport of material or the finite rate of propagation of a signal or condition. Note: Also termed transport lag and transportation lag.

4. in thermal insulation, preformed narrow blocks attached around large pipes or vessels by wires or bands.