In data communications, a kilobit is a thousand (103) bits. It's commonly used for measuring the amount of data that is transferred in a second between two telecommunication points. Kilobits per second is usually shortened to Kbps.*
Some sources define a kilobit to mean 1,024 (that is, 210) bits. Although the bit is a unit of the binary number system, bits in data communications are discrete signal pulses and have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two. See kilobyte, etc.
Note: In international English outside the U.S., the equivalent usage is "kbps" or "kbits s-1."