This document discusses human perception of pitch in audio content. It defines pitch as a subjective auditory attribute related to but not equal to frequency. Pitch is described as having two dimensions: tone height, which increases monotonically with fundamental frequency, and tone chroma, which groups tones by octave into the same pitch class. Several models for the non-linear relationship between perceived pitch and physical frequency are presented, including Mel, Bark, and cochlear maps. The concept of virtual pitch is also introduced to explain perception of pitch in complex tones even when the fundamental frequency is missing.