A reference tool where a series of choices between alternative characters leads progressively to the identification of the species.
A dichotomous key is usually a written device constructed from a series of highly organized statements arranged into couplets. A couplet consists of (typically) two descriptions which should represent mutually exclusive choices (often it is a particular combination of characteristics that determines the difference). Both choices are read and compared with the specimen to be identified. Once a decision is made, that selection directs you to another couplet (either the next in order or one further on in the key), and this process is repeated until a conclusion (successful identification) is reached.
Word origin: Latin dichotomos, Greek dichótomos.
Compare: polytomous key