If you ask me? Mobile/WiFi internet… The way and amount of time we use our phones had changed A LOT since their diffusion. I guess the release of the iPhone changed our idea of what a phone is too
Edit: when I say modern world I’m referring to the last 50 years. So stuff like “the electricity” or “the telephone” doesn’t count.
In the last century? The diode, aka the P/N junction and every variant that has been created ever since.
Recently? Capacitive touch screens are by far the most significant change.