pericyclicPronunciation: /ˌpɛrɪˈsʌɪklɪk, -ˈsɪk-/Definition of pericyclic adjective 1 Chemistry relating to or denoting a reaction that involves a concerted rearrangement of bonding in which all the bonds broken or formed in the reaction lie on a closed ring, whether or not a cyclic molecule is involved. 2 Botany relating to a pericycle.