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