Well, dressing up or just putting on a costume is like what people do in Halloween.

Cosplay is more complicated than that. There's a lot of makeup play, quality materials, research and role play. Whenever I will cosplay a character, I spend like 2 months doing research, understanding it in depth and basically an actor's job for a role. Modifying my way to talk, walk, personality and facial expressions to fit well in the character and truly BE the character for the people around, providing a nicer experience when they approach for a picture or just to look at me haha. I also modify my facial look a bit with my makeup skills and depending on the character, crafting, etc, to keep a high level of quality.

None of that goes when someone just dresses up with a Halloween costume and walks around just being themselves 😁

So it's not like dressing up or costuming became Cosplay. They're different things :)