Incest is the illegal practice of indulging in sexual activity with blood relations; that is, with members of the immediate family. In many cultures, it is also considered to be incest to indulge in these practices within the extended family, as far out as cousins.
Generally, the taboo is stronger the closer the relationship and the farther apart the ages. Thus, the taboo against parent/child incest is strongest, whereas (for example) many people would not even apply the word to sex with a step-cousin.
Historically, many cultures have made the distinction that sex or marriage with "cross cousins" is permitted, but with ortho-cousins is incest. The precise definition of cross cousin varies from culture to culture but, for example, in some cultures it would be alright to have sex with your father's sister's offspring or your mother's brother's offspring, but not your father's brother's offspring or your mother's sister's offspring. In some cultures, this goes further, requiring that the parent in question be of the opposite sex from the child (so, for example, a man could marry his mother's brother's daughter, but other combinations would not be allowed). ,