Tropism is the affinity of a virus to invade a particular cell type

Tropism
Co-receptor tropism is defined as the ability of a particular HIV-1 virus to infect a target cell using a specific co-receptor. The HIV-1 viruses can be characterized into four broad classifications based on their tropism status.

CCR5-tropic: Viruses or virus populations that can use only the CCR5 chemokine co-receptor to infect CD4+ cells.

R5 HIV

Uses the CCR5 co-receptor ONLY

CXCR4-tropic: Viruses or virus populations that can use only the CXCR4 chemokine co-receptor to infect CD4+ cells.

X4 HIV	Uses the CXCR4 co-receptor ONLY

Dual (D)-tropic: Viruses or virus populations that can use either the CCR5 or CXCR4 co-receptors to infect CD4+ cells.

Dual HIV	Can use either co-receptor

Mixed (M)-tropic: Virus populations that may contain various combinations of R5 virus, X4 virus, and/or dual-tropic viruses.

Mixed (M) HIV	Can use either co-receptor