1. A computer-implemented method comprising: identifying, from among a set of query terms, a particular query term that (i) does not occur in a lexicon of terms, and (ii) has no designated, canonical phonetic representation in a pronunciation phonetic dictionary, wherein a canonical phonetic representation comprises a sequence of phonemes; generating a phonetic representation estimate for the particular query term that (i) does not occur in the lexicon of terms, and (ii) has no designated, canonical phonetic representation in the pronunciation phonetic dictionary; transmitting data identifying at least a portion of a term that does occur in the lexicon of terms and the particular query term to a spelling correction server; receiving, from the spelling correction server, data that specifies a spelling correction confidence score, wherein the spelling correction confidence score reflects a probability that the term that does occur in the lexicon of terms is a correct spelling of the particular query term; determining that the spelling correction confidence score satisfies a predetermined threshold; and in response to determining that the spelling correction confidence score satisfies a predetermined threshold, designating, by one or more computing devices, the phonetic representation estimate for the particular query term as a canonical phonetic representation, in the phonetic dictionary, of the term that does occur in the lexicon of terms.