I thought instances is what people called Apps like Primal, Damus, and Amethyst, etc.
I could be mistaken 😅, not familiar enough with the Nostr technicals enough yet.
Discussion
That's called either app or client.
Instances are a #fediverse concept, from apps like Mastodon and works a bit different than relays here.
The main advantage on #nostr is that users can freely choose relays and have multiple ones, which makes censorship (e.g. through an instance admin) way harder