A petal (from Ancient Greek petalon "leaf", "thin plate") is one member or part of the corolla of a flower. The corolla is the name for all of the petals of a flower; the inner perianth whorl, term used when this is not the same in appearance (color, shape) as the outermost whorl (the calyx) and is used to attract pollinators based on its bright color. It is the inner part of the perianth that comprises the sterile parts of a flower and consists of inner and outer tepals. These tepals are usually differentiated into petals and sepals. The term "tepal" is usually applied when the petals and sepals are similar in shape and color. In a "typical" flower the petals are showy and colored and surround the reproductive parts. The number of petals in a flower (see merosity) is indicative of the plant's classification: eudicots (the largest group of dicots) having typically four or five petals and monocots and magnoliids having three, or some multiple of three, petals.

image:Tulip Tulipa clusiana 'Lady Jane' Rock Ledge Flower 2000px.jpg The genetics behind the formation of petals, in accordance with the ABC model of flower development, are that sepals, petals, stamens, and carpels are modified versions of each other. It seems that the mechanisms to form petals evolved a small number of times (perhaps only once), rather than evolving independently from stamens in a large number of plants.

There exists considerable variation in form of petals among the flowering plants. The petals can be united towards the base, forming a floral tube. In some flowers, the entire perianth forms a cup (called a calyx tube) surrounding the gynoecium, with the sepals, petals, and stamens attached to the rim of the cup.

The flowers of some species lack or have very much reduced petals. These are often referred to as apetalous. Examples of flowers with much reduced perianths are found among the grasses.

Other structures which may look like petals

Some plants have petaloid stamens, in plants like Canna that have true petals and staminodes, the stamen (staminodes) are modified to look like large showy petals.

A number of plants have bracts that resemble petals for example in Bougainvillea and Cornus florida (flowering dogwood). Petal-like bracts are common features in some plant families like Euphorbiaceae.

In many plants of the aster family such as the sunflower, Helianthus annuus, the circumference of the flower head is composed of ray florets. Each ray floret is anatomically an individual flower with a single large petal.