The error rate or error frequency is the ratio of the number of data transmitted with errors to the total number of data transmitted. If the information unit of the data is a bit, byte, block or frame, then this is the bit error rate( BER), the byte or block error rate( BLER) or the packet error rate( FER).
There is a direct relationship between some of the error rates mentioned, which can be determined by the probability.
The error rate is a relative frequency determined within a certain finite measurement time. A bit error frequency of 1 means that every bit is wrong. A bit error rate of `6*10^-6` means that on average 6 bits can be wrong when 1 million bits are transmitted.