1) Noise Margin: It is a measure of noise immunity of a gate or circuit (noise immunity is the ability of a gate or circuit to tolerate any noise present in a signal without performing a wrong operation)
2) Power fanout : It is the maximum number of load gates that can be connected at output without loading with same IC family and by maintaining its output within the specified limit.
The ability to drive the similar number of gates.
3) Skew: The clock signal, which is said to be applied simultaneously to all the flip-flops, may cause a minute delay changes due to some variation in the wiring between the components. Due to this, it may happen that the clock signal may arrive at the clock inputs of different flip-flops at different times. This delay is termed as skew.
The difference in the clock arrival time is call clock skew.
4) Meta stability: Metastability in electronics is the ability of a non-equilibrium electronic state to persist for a long (and theoretically unboundable) period of time.
A metastable state is half way between logic “0” and logic “1” .It is undefined state.