The performance of a coded communication system is in general measured by its probability of decoding error (called the error probability) and its coding gain over an uncoded system that transmits informatin at the same rate. There are two types of error probability, probability of word (or block) error and probability of bit error. The probability of word error is defined as the probability that a decoded word (or block) at the output of the decoder is in error. This error probability is commonly called the word-error rate (WER) or block-error rate (BLER). The probability of bit error, also called the bit-error rate (BER), is defined as the probability that a decoded information bit at the output of the decoder is in error. A coded communication system should be designed to keep these two error probability as low as possible under certain system constraints, such as power, bandwidth, and decoding complexity.
(p. 15-16, Error Control Coding - 2nd Edition - Shu Lin, Daniel J. Costello - Pearson Education International)
