| Term | Definition |
| triangle | a plane figure having three sides and three angles |
| tribe | originally, one of the three groups into which the Romans were divided, now a group of people united by the same race and customs |
| trident | a long thre-pronged (toothed) spear |
| triennial | occuring every three years |
| trilateral | having three sides |
| trilingual | speaking three languages |
| trilogy | three literary, dramatic, or musical compositions that, though each is complete in itsself, form a related series |
| trinity | a set of three persons or things that form a unit, as the three devine persons of Christian theology |
| trio | any three people or things joined or associated |
| tripartite | composed of three parts; shared by three parties |
| triplets | three children born at one birth |
| triplicate | threefold; one of three identical copies or things |
| tripod | a three-legged stand for supporting a camera or other instrument |
| trivet | a three-legged stand for holding a vessel of dish |
| tri | three |
| dent | tooth |
| enn | year |
| later | side |
| lingu | language |
| plic | to fold |
| pod | foot |