In NCAA-speak, an "official visit" means that it's D1 (or D2?), that expenses are being paid, and there are limits about how many a player can take.
In D3 coach-speak, an "official" visit probably means that coach has invited the player, set up a schedule that might include an overnight with players, meetings with coaches, etc. No travel is paid (maybe some meals?), and there are no NCAA rules about how many visits you can take. But I would guess the "official" is contrasted with an "unofficial" visit where the player shows up to tour the school without an invitation, schedule, etc.