A limerick is just a humorous or nonsense poem with a particular rhyming pattern and beat. It doesn't have to be a parody or be about a particular subject.
It has five lines with the following beats:
Lines 1, 2 and 5 rhyme and 3 and 4 rhyme.
There was a young woman named Bright
Whose speed was much faster than light.
She set out one day
In a relative way,
And returned on the previous night.