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.
Antibiotics.
Their discovery and development is what has directly enabled our sudden rise from 2B to 8B humans in only about a century. Without antibiotics, we would likely still be under 3B humans world-wide. Yes, disease really did kill off a lot of humans back in the day.
Graph out the human population over the last two centuries, and you can even see the very decade when Penicillin use became widespread, along with doctors washing their hands and other basic hygiene tasks.