Monocotyledons (monocots for short) are flowering plants (that is angiosperms, Division Magnoliophyta) whose seed contains only one embryonic leaf or cotyledon. The monocotyledonous plants are usually treated as a class, originally called Monocotyledoneae, but more recently called Class Liliopsida after the type genus, Lilium. Flowering plants that are instead dicotyledonous — having two embryonic leaves — are placed in the Class Magnoliopsida. See "How to distinguish a monocot from a dicot" for other characteristics that separate these two groups of flowering plants.
The grasses (Family Poaceae) are the second largest but most notable monocot family. Although perhaps appearing rather primitive, grasses are in fact highly evolved for wind pollination. On the other hand, orchids (Family Orchidaceae; the largest moncot family) have evolved in a different direction: becoming insect pollination specialists, with many species having the most complex and advanced flower structures in the Magnoliophyta.