1. An uncommon or unusual variant of something, particularly in linguistics, biology, or general usage
The word 'thee' is a rare form of the English pronoun 'you'.
A palavra 'thee' é uma forma rara do pronome inglês 'you'.
2. In linguistics and grammar: an infrequent inflection or conjugation of a word
The subjunctive mood contains many rare forms in modern Spanish.
O modo subjuntivo contém muitas formas raras no espanhol moderno.
3. In biology or medicine: an atypical or unusual variant of a species or condition
Scientists discovered a rare form of the virus in the patient's sample.
Cientistas descobriram uma forma rara do vírus na amostra do paciente.