Zeyd is an Iranian/Persian slang term that is used to refer to a female lover or mistress but today means girlfriend, boyfriend, or a general sexual partner. However, it's important to note that Zeyd is different from the terms "doosdokhtar" and "doospesar," which are more commonly used for casual sexual partners. Zeyd is considered a higher-level term, similar to "yar" or "refiq," not just a casual term for a sexual partner.