| Term | Definition |
| trilogy | A group of three leterary of musical works that have a related theme. |
| trisect | To divide into three parts. |
| triumvirate | A group of three, especially in authority. |
| quadrant(1) | (mathematics) A quarter of a circle or of its circumference. |
| quadrant(2) | An early machine for measuring altitudes. |
| quadrant(3) | Any of the four parts of an area divided by perpendicular lines. |
| quartet(1) | A musical composition for four voices or instruments. |
| quartet(2) | A set of four, especially of four musicians. |
| quatrain | A stanza or group of four lines of poetry. |
| decimate(1) | To destroy a large part of. |
| decimate(2) | To kill one in every ten. |
| decathlon | An athletic contest in which each contestant takes part in ten events. |
| bicentennial(n) | A two-hundredth anniversary. |
| bicentennial(adj) | Happening every 200 years. |
| centenary(adj.) | Pertaining to a 100-year period. |
| centenary(n) | A centennial. |
| centigrade | Referring to a thermometer scale of 100 degrees where water freezes at 0 degrees and boils at 100 degrees. |