the main inflected forms of a verb, from which all other inflections may be deduced. In English they are generally considered to consist of the third person present singular, present participle, past tense, and past participle
2.
the sides and interior angles of a triangle
principal parts in American English
the principal inflected forms of a verb, from which the other forms may be derived: in English, the principal parts are the infinitive, past tense, and past participle (Ex: drink, drank, drunk; go, went, gone): the present participle, derived from the present infinitive with the addition of -ing (sense 1), is sometimes regarded as one of the principal parts