16. A system, comprising: one or more processors; and memory having instructions stored thereon, the instructions, when executed by the one or more processors, cause the processors to perform operations comprising: receiving a training corpus comprising a collection of complete input phrases that span a plurality of semantically distinct domains; for each of a plurality of distinct words present in the collection of complete input phrases, calculating a respective word indexing power across the plurality of domains based on a respective normalized entropy for said word, wherein the respective normalized entropy is based on a total number of domains in which said word appears and how representative said word is for each of the plurality of domains; for each complete input phrase in the collection of complete input phrases, calculating a respective phrase indexing power across the plurality of domains based on an aggregation of the respective word indexing powers of all constituent words of said complete input phrase; obtaining respective domain-specific usage frequencies of the complete input phrases in the training corpus; and generating a cross-domain ranking of the collection of complete input phrases based at least on the respective phrase indexing powers of the complete input phrases and the respective domain-specific usage frequencies of the complete input phrases.
20. The system of claim 16 , wherein generating the cross-domain ranking of the collection of complete input phrases further comprises: calculating a respective integrated ranking score R j for each complete input phrase P j of the collection of complete input phrases based on a respective formula R j = ω v ⁢ v j + ω μ ⁢ μ j ω v + ω μ , wherein v j is a respective frequency of the complete input phrase P j that has been normalized for cross-domain comparison, μ j is a respective phrase indexing power of the complete input phrase P j across the plurality of domains, ω v and ω μ are relative weights given to domain-specific frequency and cross-domain indexing power in the ranking of the collection of complete input phrases; and generating the cross-domain ranking of the collection of complete input phrases based on the respective integrated ranking scores for the collection of complete input phrases.