A sepal (from Latin separatus "separate" + petalum "petal"^{[citation needed]}) is a part of the flower of angiosperms (flowering plants). Collectively the sepals form the calyx, which is the outermost whorl of parts that form a flower. Usually green, sepals have the typical function of protecting the petals when the flower is in bud. Another common function of sepals is protection from insect attack, and when there is no such protection, there often is some other ecological strategy, such as poison.^{[citation needed]}

Morphologically both sepals and petals are modified leaves. The calyx (plural, calices, the sepals) and the corolla (the petals) are the outer sterile whorls of the flower, which together form what is known as the perianth.

The term tepal is usually applied when the parts of the perianth are difficult to distinguish, e.g. the petals and sepals share the same color, or the petals are absent and the sepals are colorful. Examples of plants in which the term tepal is appropriate include genera such as Aloe and Tulipa. In contrast, genera such as Rosa and Phaseolus have well-distinguished sepals and petals.

Commonly in plants such as Tulipa, that have tepals, the sepals are hardly more robust than the petals, and such plants either come from warm temperate zones, or flower mainly in mild seasons, such as late spring.^{[citation needed]}