What term describes major and minor keys with the same letter names, such as C Major and c minor?

The term that describes major and minor keys with the same letter names is known as parallel keys. This concept refers specifically to pairs of keys that share the same tonic note but differ in mode; for example, C Major and C minor. Both keys start on the note C, which is the tonic for each key, but they utilize different scales and tonalities—C Major follows a major scale structure, while C minor follows a minor scale structure.

Parallel keys are important in music because they allow composers to explore different emotional qualities and textures while maintaining a connection through the tonic note. This can create contrasts in mood and atmosphere within compositions, making them a valuable tool in a musician's toolkit.

Modulating keys refers to the change of keys within a piece, while counterpoint deals with the interaction between two or more independent melodic lines. Relative keys, on the other hand, refer to pairs of keys that share the same key signature but have different tonic notes and are a major/minor pair, such as C Major and A minor. In the case of C Major and C minor, since they share the same tonic, the correct classification is parallel keys.