terti-, tern-, ter- +

(Latin: third; three each; three times)

Of the ratio of 4 to 3.
1. Consisting of, or involving, three; threefold; triple.
2. Third in order or rank.
3. Based on the number three.
4. Pertaining to, consisting of, compounded of, or characterized by a set (or sets) of three; threefold, triple.

A ternary system (of classification), one in which each division is into three parts.

A woman who is pregnant for the third time.
A female which has had three pregnancies that resulted in viable (living) offspring.
tertium quid
1. Some third thing; a third something.
2. An unknown or indefinite thing or factor that is related to but cannot be classified as belonging to either of two other areas or categories.
3. Something in between two fixed points or positions; a third alternative or choice beyond two fixed choices.

Cross references of word families that are related, partially or totally, to: "three, third": terce-; tri-, tre; trigono-; trito-.