Is what the questioner did—imagining having sexual intercourse with a specific person while masturbating due to obsessive thoughts—considered fornication (zina), a major sin (kabira), or a minor sin (lamam)?

It is not permissible to deliberately conjure up sexual fantasies and derive pleasure from them. However, if they occur spontaneously without being sought, then there is no blame. Masturbation by hand is forbidden and is not considered actual fornication (zina) or a major sin. However, intentionally imagining intercourse with someone who is unlawful to you and then performing masturbation, even if they are minor sins, should not be taken lightly, because persistence in a minor sin can turn it into a major one.