Although for most practical purposes, the pitch of a sound can be said to be simply a measure of its frequency, there are circumstances in which a constant frequency sound can be perceived to be changing in pitch.
One of most consistently observed "psychoacoustic" effects is that a sustained high frequency sound (>2 kHz) which is increased steadily in intensity will be perceived to be rising in pitch, whereas a low frequency sound (<2 kHz) will be perceived to be dropping in pitch.
The perception of the pitch of short pulses differs from that of sustained sounds of the same measured frequency. If a short pulse of a pure tone is decaying in amplitude, it will be perceived to be higher in pitch than an identical pulse which has steady amplitude. Interfering tones or noise can cause an apparent pitch shift.