A leaflet in botany

Botany

Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...

 is a part of a compound leaf

Leaf

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

. A leaflet may resemble an entire leaf, but it is not borne on a stem

Plant stem

A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence , conifer cones, roots, other stems etc. The internodes distance one node from another...

 as a leaf is, but rather on a vein of the whole leaf. Compound leaves are common in many plant families. For example, a tomato

Tomato

The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...

plant has compound leaves with leaflets.

Leaflets borne on the central vein of a leaf are referred to as pinnae; the compound leaves themselves are described as pinnate. A plant may be further subdivided in that the pinnae are themselves split into leaflets, or pinnules; these leaves are now twice pinnate, or bipinnate. A few plant species even have tripinnate leaves.