A clade is a group consisting of a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 (extinct or extant) and all its descendants. In the terms of biological systematics

Systematics

Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...

, a clade is a single "branch" on the "tree of life

Tree of life (science)

Charles Darwin proposed that phylogeny, the evolutionary relatedness among species through time, was expressible as a metaphor he termed the Tree of Life...

". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 name is central to biological classification

Biological classification

Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....

. In cladistics

Cladistics

Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

 (which takes its name from the term), clades are the only acceptable units.

The term was coined in 1958 by English biologist Julian Huxley

Julian Huxley

Sir Julian Sorell Huxley FRS was an English evolutionary biologist, humanist and internationalist. He was a proponent of natural selection, and a leading figure in the mid-twentieth century evolutionary synthesis...

.
A clade is termed monophyletic, meaning it contains one ancestor (which can be an organism, a population

Population

A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

, or a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

) and all its descendants.
A semantic case has been made that the name should be "holophyletic," but this term has not acquired widespread use.