Heterologous tissue in an organism is tissue that is foreign or taken from a different species. For example, a pig’s heart valve, transplanted into a human, is heterologous. But so are the cells of a tumor, because they’re foreign organic material.
There’s another meaning of heterologous in biology. Heterologous structures in different species are structures that don't have the same evolutionary origin. For example, even though a bird's wing and mosquito's wing are both used for flying and have a similar position on the body, they're heterologous, because they developed along separate evolutionary paths. Heterologous uses the combining form hetero-, meaning "different" or "other," from the Greek heteros.