In phylogenetics, a primitive (or ancestral) character, trait, or feature of a lineage or taxon is one that is inherited from the common ancestor of a clade (or clade group) and has undergone little change since. Conversely, a trait that appears within the clade group (that is, is present in any subgroup within the clade but not all) is called advanced or derived. A clade is a group of organisms that consists of a common ancestor and all its lineal descendants.
A primitive trait is the original condition of that trait in the common ancestor; advanced indicates a notable change from the original condition. These terms in biology contain no judgement about the sophistication, superiority, value or adaptiveness of the named trait. "Primitive" in biology means only that the character appeared first in the common ancestor of a clade group and has been passed on largely intact to more recent members of the clade. "Advanced" means the character has evolved within a later subgroup of the clade.
Cladograms are important for scientists as they allow them to classify and hypothesize the origin and future of organisms. Cladograms allow scientists to propose their evolutionary scenarios about the lineage from a primitive trait to a derived one. By understanding how the trait came to be, scientists can hypothesize the environment that specific organism was in and how that affected the evolutionary adaptations of the trait that came to be.
Other, more technical, terms for these two conditions—for example, "plesiomorphic" and "synapomorphic"—are frequently encountered; see the table below.
At least three other sets of terms are synonymous with the terms "primitive" and "advanced". The technical terms are considered preferable because they are less likely to convey the sense that the trait mentioned is inferior, simpler, or less adaptive (e.g., as in lower and higher plants). The terms "plesiomorphy" and "apomorphy" are typically used in the technical literature: for example, when a plesiomorphic trait is shared by more than one member of a clade, the trait is called a symplesiomorphy, that is, a shared primitive trait; a shared derived trait is a synapomorphy.
"Primitive" and "advanced" are relative terms. When a trait is called primitive, the determination is based on the perspective from which the trait is viewed. Any trait can be both primitive (ancestral) and advanced (derived) depending on the context.
In the clade of vertebrates, legs are an advanced trait since it is a feature that appears in the clade. However, in the clade of tetrapods, legs are primitive since they were inherited from a common ancestor.
The terms "primitive" and "advanced", etc., are not properly used in referring to a species or an organism as any species or organism is a mosaic of primitive and derived traits. Using "primitive" and "advanced" may lead to the latin term Scala Naturae or "ladder thinking"  which is the thought that all species are evolving because they are striving toward supremacy. When this form of thinking is used, humans are typically considered perfect and all other organisms are of less quality than them. This can cause the misconception of one species being an ancestor to another species, when in fact both species are extant. Homo sapiens, for example have large brains (a derived trait) and five fingers (a primitive trait) in their lineage. Species are constantly evolving, so a frog is not biologically more primitive than a human as each has been evolving continuously since each lineage split from their common ancestor.
In cladistics or phylogenetics, an outgroup is a more distantly related group of organisms that serves as a reference group when determining the evolutionary relationships of the ingroup, the set of organisms under study, and is distinct from sociological outgroups. The outgroup is used as a point of comparison for the ingroup and specifically allows for the phylogeny to be rooted. Because the polarity (direction) of character change can be determined only on a rooted phylogeny, the choice of outgroup is essential for understanding the evolution of traits along a phylogeny.