Noise margin

In electrical engineering, noise margin is the maximum voltage amplitude of extraneous signal that can be algebraically added to the noise-free worst-case input level without causing the output voltage to deviate from the allowable logic voltage level. It is commonly used in at least two contexts as follows:
In practice, noise margins are the amount of noise, that a logic circuit can withstand.
Noise margins are generally defined so that positive values ensure proper operation, and negative margins result in compromised operation, or outright failure.