Short for Maximum Segment Size. The maximum segment size (MSS) is the largest amount of data, specified in bytes, that a computer or communications device can handle in a single, unfragmented piece. * Each TCP device has associated with it a ceiling on TCP size a segment size that will never be exceeded regardless of how large the current window is. This is called the maximum segment size (MSS). When deciding how much data to put into a segment, each device in the TCP connection will choose the amount based on the current window size, in conjunction with the various algorithms, but it will never be so large that the amount of data exceeds the MSS of the device to which it is sending. [* Source: Adapted from the TCP/IP Guide]
See also Maximum Transmission Unit (MTU).