trim·e·ter 
(trĭmĭ-tər)
(trĭmĭ-tər)n.
1.
a. Verse written in lines of three metrical feet.
b. A single line of such verse.
2.
a. Classical quantitative verse consisting of three measures of two feet each, especially in iambic, trochaic, or anapestic meter.
b. A single line of such verse.
[Late Latin, from Latin trimetrus, from Greek trimetros : tri-, tri- + metron, measure; see METER1.]
tri·metric (trī-mĕtrĭk), tri·metri·cal (-rĭ-kəl) adj.