A unit of 1000 Hz, or 1000 cycles-per-second, used in measuring a pitch's frequency.
A tone of 1 kHz is just below "high-C" (2 ledger-lines above the treble-staff).
The upper limit of human hearing varies with each individual and with age and amount of exposure to loud sounds, but for most people it lies somewhere within the octave between 10 and 20 kHz.
Many other animals (particularly bats and dolphins) can hear much higher frequencies; for example, dolphins can hear sounds up to 200 kHz -- this is about 4 octaves higher than the limit of human hearing. (4 octaves down = ratio 1/(2^4) = 1/16, so 4 octaves below 200 kHz is 200/16 = 12.5 kHz).