Radio telecommunications; symbol refers to a transmitted or received piece of information representing a fixed number of bits. Depending on the radio's constellation, it can represent anything from 1 to an infinite number of bits. The more complicated the constellation, the more information a symbol can represent. Obviously, more complexity means greater susceptibility to error. Bit Error Rate is an exponential relation to number of possible symbols (eg, size of constellation).
See also: constellation, spread spectrum, frequency-hopping, direct sequence, Bit Error Rate.