Noise figure is a commonly used system parameter that quantifies the degradation in the signal-to-noise ratio (SNR) as the signal passes through a receiving system. Because of the difficulty in defining the SNR, NF depends on how the SNR is computed and the underlying assumptions that are made. Existing NF measures and their shortcomings are explained. A new NF suitable for digital communication receiver is proposed by redefining the SNR, so that the NF measures the degradation in the achievable performance caused by the receiving system. The proposed NF, which we refer to as the effective NF, can be readily determined based on the existing NF measurement techniques.