The term frond refers to a large, divided leaf. In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group. Other botanists allow the term frond to also apply to the large leaves of cycads and palms (Arecaceae).
When most people use the word frond they mean a large, compound leaf, but if the term is used botanically to refer to the leaves of ferns, it may be applied to smaller and undivided leaves.
Fronds, like all leaves, usually have a stalk called the petiole supporting a flattened blade, sometimes called a lamina. However, fronds are often described using distinctively different terms. The petiole of a frond is called a stipe and the continuation of the stipe into the blade portion is called the rachis. The blades may be simple (undivided), pinnatifid (deeply incised, but not truly compound), pinnate (compound with the leaflets arranged along a rachis to resemble a feather). If a frond is pinnate, the segments of the blade are called pinnae (singular: pinna) and the stalks bearing the pinnae are called petiolules (The main vein or mid-rib of a pinna is sometimes called a costa (pl., costae).
If a frond is divided into pinnae, the frond is called once pinnate. In some fronds the pinna are further divided into segments, creating a bipinnate frond. The segments into which each pinna are divided are called pinnules. Rarely, a frond may even be tripinnate, in which case the pinnule divisions are known as ultimate segments.
Pinnae may be arranged along the rachis either directly opposite one another or alternating up the stem. The arrangement may change from the base of a blade to the tip, as in the example of Blechnum shown below (from base to tip: pinnae opposite to alternate, and pinnatisect to pinnatifid).
Some fronds are not pinnately compound (or simple), but may be palmate or bifurcate. Some ferns, like members of the group Ophioglossales have a unique arrangement.
Fern fronds often bear sporangia, usually on the abaxial surface of the pinnae, but sometimes marginally or scattered over the frond. The sporangia are typically clustered into a sorus (pl., sori). Associated with each sorus in many species is a membranous protective structure called an indusium: an outgrowth of the blade surface that may partly cover the sporangia. Fronds may bear hairs, scales, glands, and, in some species, bulblets for vegetative reproduction.
Fern fronds, as with all leaves, arise from the stem, either directly, or on an outgrowth from the stem termed a phyllopodium. The stem of a typical (leptosporangiate) fern is subterranean or horizontal on the surface of the ground. These stems are called rhizomes. Many fern fronds are initially coiled into a "fiddle-head" or "crozier" (see circinate vernation) although cycad and palm fronds do not have this type of vernation.
Some fern species feature frond dimorphism, in which fertile and sterile fronds differ in appearance and structure.