Amaryllis (the name of a shepardess in classical Greek poetry) is a plant genus containing the belladonna lily (Amaryllis belladonna), a native of South Africa, which was introduced into cultivation at the beginning of the 18th century. This is a half-hardy bulbous plant, producing in the fall a number of strap-shaped, dull green leaves, 1-1 1/2 ft. long, arranged in two rows which it eventually loses by late spring and goes dormant until late summer/early fall, when it produces one or two naked stems, each bearing at the top a cluster of 2-12 funnel-shaped flowers. This flowering pattern is the cause of its common name 'naked lady'. Most of the so-called Amaryllis sold as 'ready to bloom for the holidays' bulbs belong to the allied genus Hippeastrum, despite being labeled as 'Amaryllis' by sellers and nuseries.
Continuing the theme of name confusion, bulbs of other species with a similar growth/flowering pattern sometimes are called 'naked ladies', even though those species have their own more widely used and accepted common names, such as 'resurrection lily' for Lycoris squamigera for example.
Note: The flower currently pictured on this page is actually a Hippeastrum, not an Amaryllis.