Oh, reference and anything else I think of I'll put here.
Definition: limerick, n. A kind of humorous verse of five lines, in which the first, second, and fifth lines rhyme with each other, and the third and fourth lines, which are shorter, form a rhymed couplet.
Dictionary definition is missing some specifics. Generally is anapestic (two unstressed syllables followed by a stressed one) with lines 1, 2, and 5 having three such feet (measuring units) and lines 3 and 4 having two feet. Of course there are exceptions (a line may begin by dropping a weak unstressed syllable and adding one to the end instead).
In the current draft, limerick 1 line 2 is in reference to Dylan Thomas's "Do not go gentle into that good night." Too subtle? Too sensitive to make a joke about copyright?