Annulen
Annulene refers to cyclic hydrocarbons in which double bonds and single bonds alternate. When naming an annulene, n is the number of carbon atoms constituting the molecule and is expressed in the manner of 'annulene'. In this case, n can be 4, 6, 8, 10, or even 4 or more. Or systematic names such as cyclobutadiene, cyclooctatetraene, and the like.