Acronym for single in-line memory module, a small circuit board that can hold a group of memory chips. Typically, SIMMs hold up 8 (on Macintoshes) or 9 (on PCs) RAM chips. On PCs, the ninth chip is for parity error checking. Unlike memory chips, SIMMs are measured in bytes rather than bits. In todays SIMMs, each chip holds 2MB, so a single SIMM holds 16MB. SIMMs are easier to install than individual memory chips.