All terms
Whether something is recorded on the blockchain itself or stored somewhere else.
Onchain means the data or the action is recorded in the blockchain state, visible to everyone and permanent. Offchain means it happens somewhere else: a server, a database, a layer 2 before it settles, or a file store.
Most real applications mix the two, because putting everything onchain is prohibitively expensive. The interesting question is always which parts are onchain, because those are the parts nobody can quietly change.
An NFT ownership record is onchain. The image it points at is almost always offchain, which is why storage choice matters.
That onchain automatically means trustless. Data being onchain proves it was published, not that it is true. An oracle can write a wrong price onchain and the chain will record it faithfully.
Common questions
Why is not everything onchain?
How do I check whether something is really onchain?
Where to go next
More web3 terms
Want this explained out loud?
Some of these land faster in conversation than in writing. Sessions are one to one and you can ask anything, including the things that feel too obvious to ask.