Window period in HIV is defined as
Window period                                                                       

The time period between primary infection and detection of antibodies is called window period.
Traditional methods used in the diagnosis of HIV such as (ELISA and western blot) are based on the detection of these antibodies i.e., these tests become positive once the antibodies appear in the blood.
However, the recent methods used in the diagnosis of HIV are based on the detection of virus or virus particles (not antibodies).
Virus or the virus particles appear early in the blood than antibodies so the newer methods have considerably reduced the period between the infection and the diagnosis i.e., they have reduced the window period

Methods of the detection                                             Window period

Antibody detection (ELISA western blot) →               22 days
p24 Antigen   →                                                          16 days
Nucleic detection →                                                     12 days
Period between onset of infection  and clinically detectable level of antibodies
Period between infection and appearance  of symptoms
Time between HIV positivity to AIDS
CD4 count < 200