TLDR: Anything in Active Inventory or originating from your stock is considered an item. Gifting/transferring an item doesn't count unless that item's specific effect was proc'd.
This question is very tough- the only two ways to 100% be certain is to play through multiple times keeping track of what you use, or two use a mod tool to view where the game stores 'items used.'
I looked at several sources for what Kingdom Hearts defines as an 'item,' and I found several references that corroborate what I had suspected- Items are defined as anything that can be stored in stock or active inventory, and item use is only proc'd if that item's effect is initialized (not necessarily triggered).
First, I found an old IGN guide (here) that items must be used from "the sub-menu" and are stored in the main stock, and must then be shifted to the active inventory.
Next, I found a wonderful guide put together by /hardcore rpg gamer/ (here) that itemized the types of items you may encounter in the game:
- AP Boost
- Defense Boost
- Drive Recovery
- High Drive Recovery
- Magic Boost
- Power Boost
- Torn Page
Lastly, I looked up the definition in the Item Page of the original Strategy Guide for Kingdom Hearts (found the PDF on a old site that I will not post due to copyright issues). It defined items somewhat nebulously, referring to weapons occasionally as items, yet having separate pages for both items and weapons. However, on the main item page it says: "Items may be stored in your main stock, your active inventory, or the inventory of a party member." I believe this fits my definition.
Anything in Active Inventory or originating from your stock is considered an item. Gifting/transferring an item doesn't count unless that item's specific effect was proc'd. In addition, keyblades, synthesis items, and accessories do NOT count as items used, but summon charms do.