Wiktionary
alt. (context linguistics English) The placing of the topic of a sentence at the beginning. n. (context linguistics English) The placing of the topic of a sentence at the beginning.
WordNet
n. (linguistics) emphasis placed on the topic or focus of a sentence by preposing it to the beginning of the sentence; placing the topic at the beginning of the sentence is typical for English; "`Those girls, they giggle when they see me' and `Cigarettes, you couldn't pay me to smoke them' are examples of topicalization"
Wikipedia
Topicalization is a mechanism of syntax that establishes an expression as the sentence or clause topic: in English, by having it appear at the front of the sentence or clause (as opposed to in a canonical position further to the right). Topicalization often results in a discontinuity and is thus one of a number of established discontinuity types (the other three being wh-fronting, scrambling, and extraposition). Topicalization is also used as a constituency test; an expression that can be topicalized is deemed a constituent. The topicalization of arguments in English is rare, whereas circumstantial adjuncts are often topicalized. Most languages allow topicalization, and in some languages, topicalization occurs much more frequently than in English.