Dichotomous (outcome or variable) means “having only two possible values”, e.g. “yes/no”, “male/female”, “head/tail”, “age > 35 / age <= 35” etc.
For example, the outcome of an experiment with coin tossing is dichotomous (“head” or “tail”); the variable “biological sex” in a social study is dichotomous (“male” or “female”).
Dichotomous variables are the simplest and intuitively clear type of random variables. For this reason mental (and real) coin-tossing experiments are often used in introductory courses in statistics and probability. Nevertheless, statistical methods developed for analysis of dichotomous variables are often more complex, both conceptually and mathematically, than parallel methods for continuous variables.