(1) A user-replaceable module in a computer that holds RAM chips. "Memory card" mostly refers to flash chips in definition #2 below. See memory module.
(2) A removable module that contains non-volatile flash memory chips used for storage in digital cameras and mobile devices. The major card types are SD cards, Memory Sticks and CompactFlash; the SD card family being the most popular. See flash memory, memory card reader, SD card, Sony Memory Stick and CompactFlash.
Flash-based memory cards are 21st century "film." This digital SLR camera uses SD cards, the most popular memory card type.
Ever since digital cameras came on the scene in the late 1990s, a variety of memory cards have been developed.