There is a notation that had an immediate and profound impact on research in algebraic topology, later algebraic geometry, and was eventually adopted by all areas of pure mathematics: the introduction of arrows to denote mappings. Compare $f \colon X \to Y$ with $f(X) \subset Y$, which is what was used previously. 

The use of arrows led to commutative diagrams, without which many parts of modern mathematics are now inconceivable. I mentioned this before in an answer [here][1].

[1]: https://math.stackexchange.com/questions/144821/who-came-up-with-the-arrow-notation-x-rightarrow-y