1. A system comprising: a classifier implemented in one or more computers, comprising: an embedding function layer configured to receive an input comprising a plurality of words that surround an unknown word in a sequence of words and map the plurality of words into a numeric representation in a high-dimensional space; and a classifier layer configured to process the numeric representation of the input to generate a respective word score for each word in a pre-determined set of words, wherein each of the respective word scores represents a predicted likelihood that the corresponding word is the unknown word; and instructions to process each word in a vocabulary of words using the embedding function layer to obtain a respective numeric representation of each word in the vocabulary in the high-dimensional space and to associate each word in the vocabulary with the respective numeric representation of the word in the high-dimensional space.