window period

In medicine, the window period for a test designed to detect a specific disease (particularly infectious disease) is the time between first infection and when the test can detect that infection. In antibody-based testing, the window period is dependent on the time taken for seroconversion.The window period is important to epidemiology and safe sex strategies, and in blood and organ donation, because during this time, an infected person or animal cannot be detected as infected but may still be able to infect others. For this reason, the most effective disease-prevention strategies combine testing with a waiting period longer than the test's window period.
See more at Wikipedia.org...

the time between primary infection and the appearance of antibodies against an organism (seroconversion).

The time period between a person's infection with HIV and the appearance of detectable anti-HIV antibodies. Because antibodies to HIV take some time to form, an HIV antibody test will not be positive immediately after a person is infected. The time delay typically ranges from 14 to 21 days, but varies for different people. Nearly everyone infected with HIV will have detectable antibodies by 3 months after infection.
See Also: Enzyme-Linked Immunosorbent Assay, Antibody
  

Time from infection with HIV until detectable seroconversion<!-- (see) -->.

Refers to the time between infection with HIV and when its antibodies can be detected in the blood (as short as six weeks but usually longer, up to six months for test purposes).