[n] a verse line having four metrical feet
A tetrameter is a line of four metrical feet.
Example
Anapestic tetrameter:
"And the sheen of their spears was like stars on the sea" (Byron, "The Destruction of Sennacherib")
Iambic tetrameter:
"Because I could not stop for Death" (Emily Dickinson)
Trochaic tetrameter:
"Peter, Peter, pumpkin-eater" (English nursery rhyme)
Dactylic tetrameter:
Picture your self in a boat on a river with [...] (The Beatles, "Lucy in the Sky with Diamonds")