Generally, an automatic speech recognition system can recognise only a limited number of
words (or more generally items). The set of all words the recognition system has
been set up to be able to recognise is called the vocabulary V. Similarly, one can define the
test vocabulary 
as the set of words appearing in the evaluation
test . A word w is called out-of-vocabulary
(OOV) if it is present in the test vocabulary but not present
in the recogniser's vocabulary, i.e. if 
.