Plesiomorphy and symplesiomorphy
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In phylogenetics, a plesiomorphy and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades.
Phylogenies showing the terminology used to describe different patterns of ancestral and derived trait states.
Imaginary cladogram. The yellow mask is a plesiomorphy for each living masked species, because it is ancestral. It is also a symplesiomorphy for them. But for the four living species as a whole, it is an apomorphy because it is not ancestral for all of them. The yellow tail is a plesiomorphy and symplesiomorphy for all living species.
Apomorphy and synapomorphy
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In phylogenetics, an apomorphy is a novel character or character state that has evolved from its ancestral form. A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have evolved in their most recent common ancestor. In cladistics, synapomorphy implies homology.
Phylogenies showing the terminology used to describe different patterns of ancestral and derived character or trait states.