Vocal register

A vocal register is a range of tones in the human voice produced by a particular vibratory pattern of the vocal folds. These registers include modal voice (or normal voice), vocal fry, falsetto, and the whistle register. Registers originate in laryngeal function. They occur because the vocal folds are capable of producing several different vibratory patterns. Each of these vibratory patterns appears within a particular range of pitches and produces certain characteristic sounds.

In speech pathology, the vocal register has three components: a certain vibratory pattern of the vocal folds, a certain series of pitches, and a certain type of sound. Although this view is also adopted by many vocal pedagogists, others define vocal registration more loosely than in the sciences, using the term to denote various theories of how the human voice changes, both subjectively and objectively, as it moves through its pitch range. There are many divergent theories on vocal registers within vocal pedagogy, making the term somewhat confusing and at times controversial within the field of singing. Vocal pedagogists may use the term vocal register to refer to any of the following:

Manuel Garcia II was one of the first to develop a scientific definition of registers, a definition that is still used by pedagogues and teachers today. "A register is a series of homogeneous sounds produced by one mechanism, differing essentially from another series of equally homogeneous sounds produced by another mechanism." Another definition worth considering is from Clifton Ware. "A series of distinct, consecutive, homogeneous vocal tones that can be maintained in pitch and loudness throughout a certain range." A Register, consists of the homogeneous tone qualities produced by the same mechanical system whereas Registration is the process of using and combining the registers to achieve artistic singing. For example: a skilled singer moves through their range and dynamics smoothly, so that you are unaware of register changes. This process could be described as good or clean registration. The term “register” came about in the sixteenth century. Before then it was recognized that there were different “voices”. As teachers started to notice how different the areas on either side of the passaggios or breaks in the voice were, it was compared to the different sets of pipes in an organ. These clusters of pipes were called registers, we now use the same term when speaking of voices.

...
Wikipedia