1.
If something changes substantially or is substantially different, it changes a lot or is very different. (FORMAL)
The percentage of girls in engineering has increased substantially...
The price was substantially higher than had been expected...
= significantly
ADV: ADV with v, ADV adj/prep
2.
If you say that something is substantially correct or unchanged, you mean that it is mostly correct or mostly unchanged. (FORMAL)
He checked the details given and found them substantially correct...
ADV: ADV adj