Byronik's note: Viral tropism is the specificity of a virus for a particular host tissue, determined in part by the interaction of viral surface structures with receptors present on the surface of the host cell.

And that explanation was about as clear as mud to me until I read this: “A tropism test is a blood test that reveals how your HIV enters T-cells. HIV can enter T-cells via an R5 or X4 co-receptor. A tropism test reveals which of these receptors your HIV uses.”