INVITATORY is a 10 letter word that starts with I.

1. \In*vi"ta*to*ry\, a. [L. invitatorius: cf. F.
   invitatoire.]
   Using or containing invitations.
   
         The ``Venite'' [Psalm xcv.], which is also called the
         invitatory psalm.                        --Hook.
   
   

2. \In*vi"ta*to*ry\, n.; pl. {Invitatories}. [LL.
   invitatorium: cf. F. invitatoire.]
   That which invites; specifically, the invitatory psalm, or a
   part of it used in worship.