Acquired immune deficiency syndrome. The late stages of HIV. “A person with HIV is considered to have progressed to AIDS when: the number of their CD4 cells falls below 200 cells per cubic millimeter of blood (200 cells/mm3). (In someone with a healthy immune system, CD4 counts are between 500 and 1,600 cells/mm3.) OR they develop one or more opportunistic infections regardless of their CD4 count.”
For more information, the U.S. healthcare system offers a handy primer.
https://www.hiv.gov/hiv-basics/overview/about-hiv-and-aids/what-are-hiv-and-aids