|Definition - As defined by Steven Pinker, this is a period of several months — usually occurring in the third year — during which the child suddenly begins to speak in complex sentences, e.g. "Yes, I did kick the ball" instead of "Bobby kick ball."|
Quotation - "Children's first grammars consist only of lexical phrases built around content words. Around the age of two they acquire functional phrases built around grammatical morphemes within which they can fit their lexical phrases. This stage of language development is so rapid that it is known as 'the grammar explosion'. In fact development is more ordered than the explosion imagery (or Pinker's alternative 'All hell breaks loose') suggests."
(Source: http://pages.slc.edu/~ebj/IM_97/Lecture13/L13.html )