IPA: /ˌɪn.vɪˈteɪ.ʃən.əl/
KK: /ɪnˈvaɪtəˌnəl/
Relating to an event or competition that is only open to those who have been invited to participate.
The invitational tournament attracted some of the best players in the country.
An event, often a sports competition, that is only open to people who have been invited to participate.
The school hosted an invitational for the best high school basketball teams in the region.
Invitational is formed from "invite" (from Latin "invitare", meaning to summon or call in) and "-ional" (a suffix used to form adjectives indicating a relation to). The word "invitational" refers to something related to an invitation, typically an event where participants are invited to compete or attend.
Think of 'inviting' someone to an event — 'invite' means to summon, and '-ional' indicates a relation. This helps you remember that 'invitational' refers to something related to an invitation.