If something changes substantially or is substantially different, it changes a lot or is very different. The percentage of girls in engineering has increased substantially The price was substantially higher than had been expected = significantly
to a great extent or degree; "I'm afraid the film was well over budget"; "painting the room white made it seem considerably (or substantially) larger"; "the house has fallen considerably in value"; "the price went up substantially"
If you say that something is substantially correct or unchanged, you mean that it is mostly correct or mostly unchanged. He checked the details given and found them substantially correct