Something homologous is similar in function to something else. You might say that your dog's hind legs and your legs are homologous, because they have similar structure and function.

Homologous comes from the Latin for "agreeing," and has the root homo- in it, which means "same." In biology, homologous either refers to traits that share an origin, and have the same function (like the spine on all vertebrates), or structures that share an evolutionary origin, but have developed different functions. Either way, something is the same about them. You'll also find homologous in math, like when two triangles share a line.

Definitions of homologous
  1. adjective
    corresponding or similar in position or structure or function or characteristics; especially derived from an organism of the same species
    “a homologous tissue graft”
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    derived from organisms of a different but related species
    derived from organisms of the selfsame individual
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  2. adjective
    having the same evolutionary origin but not necessarily the same function
    “the wing of a bat and the arm of a man are homologous
    homologic, homological
    similar in evolutionary origin but not in function
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    heterologic, heterological, heterologous
    not corresponding in structure or evolutionary origin
    corresponding in function but not in evolutionary origin
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