Sample rate
How many times per second an analogue audio signal is measured when it is turned into numbers.
Each measurement is a sample, and the rate is given in hertz. CD audio runs at 44.1 kHz, film and video at 48 kHz, studios happily at 96 kHz. The odd figure of 44,100 comes from early PCM adaptors that stored audio on videotape: three samples per scan line, 245 usable lines, 60 fields a second. The standard was born out of a VCR, not out of the human ear.
The rate decides which frequencies can be reproduced — at most half of it, the Nyquist frequency. Mixing rates in one project means resampling, and two devices agreeing on the number is not enough: they also need to share a clock over word clock, or they slowly drift apart.