Tepals are elements of the perianth

Perianth

The term perianth has two similar but separate meanings in botany:* In flowering plants, the perianth are the outer, sterile whorls of a flower...

, or outer part of a flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

, which include the petal

Petal

Petals are modified leaves that surround the reproductive parts of flowers. They often are brightly colored or unusually shaped to attract pollinators. Together, all of the petals of a flower are called a corolla. Petals are usually accompanied by another set of special leaves called sepals lying...

s or sepal

Sepal

A sepal is a part of the flower of angiosperms . 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...

s. The term tepal is more often applied specifically when all segments of the perianth are of similar shape and color, or undifferentiated, which is called perigone. When different types of organs can be distinguished, they are referred to as petals and sepals.

Undifferentiated tepals are thought to be the ancestral condition in flowering plant

Flowering plant

The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...

s. Amborella

Amborella

Amborella is a genus of rare understory shrubs or small trees endemic to the island of New Caledonia. The genus consists of only a single species, Amborella trichopoda, and is the only member of the family Amborellaceae. Wood of Amborella lacks the vessels characteristic of most flowering plants...

, which is thought to have separated earliest in the evolution of flowering plants, has flowers with undifferentiated tepals. Distinct petals and sepals would therefore have arisen by differentiation, probably in response to animal pollination. In typical modern flowers, the outer or enclosing whorl

Whorl (botany)

In botany, a whorl is an arrangement of sepals, petals, leaves, or branches in which all the parts are attached at the same point and surround or wrap around the stem.There are four whorls in a general flower...

 of organs forms sepals, specialised for protection of the flower bud as it develops, while the inner whorl forms petals, which attract pollinator

Pollinator

A pollinator is the biotic agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain...

s.

In some plants the flowers have no petals, and all the tepals are sepals modified to look like petals. These organs are described as petaloid, e.g. the sepals of Hellebore

Hellebore

Commonly known as hellebores, members of the genus Helleborus comprise approximately 20 species of herbaceous perennial flowering plants in the family Ranunculaceae, within which it gave its name to the tribe of Helleboreae...

.

Undifferentiated tepals are common in Monocotyledon

Monocotyledon

Monocotyledons, also known as monocots, are one of two major groups of flowering plants that are traditionally recognized, the other being dicotyledons, or dicots. Monocot seedlings typically have one cotyledon , in contrast to the two cotyledons typical of dicots...

s. In tulip

Tulip

The tulip is a perennial, bulbous plant with showy flowers in the genus Tulipa, which comprises 109 species and belongs to the family Liliaceae. The genus's native range extends from as far west as Southern Europe, North Africa, Anatolia, and Iran to the Northwest of China. The tulip's centre of...

s, for example, the first and second whorls both contain structures that look like petals. These are fused at the base to form one large, showy, six-parted structure. In lilies

Lilium

Lilium is a genus of herbaceous flowering plants growing from bulbs. Most species are native to the temperate northern hemisphere, though the range extends into the northern subtropics...

the organs in the first whorl are separate from the second, but all look similar, thus all the showy parts are often called tepals.

Usage of the term 'tepal' is inconsistent - some authors refer to 'sepals and petals' where others use the word 'tepals' in the same context.