Learner's definition of INVITATION
1 [count] : a written or spoken request for someone to go somewhere or to do something
We sent out more than 100 invitations for the party.
He accepted/declined our invitation.
an invitation to lunch/dinner
a wedding invitation
You have a standing/open invitation [=you are always welcome; you are always invited] to visit us any time.
2 [noncount] : the act of inviting someone
She attended the fund-raiser at the invitation of the committee chairperson. [=she attended because she was invited by the committee chairperson]
The event is by invitation only. [=only people who have been invited to the event can attend it]
3 [singular] : something that encourages someone to do something or that makes something more likely to happen — + to
She treated his request for help as an invitation to take control of his life.
Leaving valuables unattended is an (open) invitation to thieves (to steal them).
His letter to the newspaper was an invitation to criticism.
Speeding is an invitation to disaster/trouble.