Sexual desire, which is often used synonymously with sex drive or libido, generally speaking refers to a person's level of interest in sexual activity as well a person's openness or receptivity to sexual pleasure.
There is no single definition of sexual desire; ask a doctor, a philosopher, an artist, an activist and you'll get different answers depending on their understanding of human experience and what's important about how we as humans function. One might say that sexual desire is biological, one might say it's social, one might say it's spiritual.
A definition of sexual desire then may be as important not for what it describes, as for how it helps us distinguish different parts of sexual experience. For example, sexual desire may be thought of as distinct from sexual arousal, or the ways we feel turned on or excited by sexual stimulation. Sexual desire may be thought of as distinct from sexual pleasure, which is to say a desire for sex is different from feeling pleasure from sex, or pleasure from that desire. We may, for example, have sexual desire that makes us feel sexual shame and not pleasure.
Sex researchers study and measure their own definitions of sexual desire by asking people questions about how interested they are in sex, and sometimes by asking other people to evaluate their partners' level of interest in sex. In most cases sexual desire is measured by asking people to make these kinds of cognitive evaluations, rather than hooking people up to machines and seeing how their body responds (although as brain research develops this is likely to change). They also have their own definitions of what is too much or too little interest in sex, referring to people as being "hypersexual" if they have a high interest in sex, and having "hypoactive" sexual desire, when their desire is deemed too low.
Of course in our own experience these things are all intimately connected. But when people feel stuck or troubled by sexual experiences, it can be helpful to try and separate out these different parts of sexual experience, if only to get some momentary clarity.