What's the difference between "veggie" and "vegetable"? Can I use them interchangeably? or is there any difference in terms of meaning and usage?
In British English, veggie means vegetarian (at least according to the Collins Dictionary). It can also be an adjective which is used to talk about food that doesn't contain any meat or fish: Going veggie can be tasty, easy and healthy too. Veg is an informal British word which means a vegetable or vegetables: I like both fruit and veg.
In American English veggie means vegetable first of all. But of course it is more informal than vegetable: ...well-balanced meals of fresh fruit and veggies, chicken, fish, pasta, and no red meat.
So, if you want to use veggie right, mind the regional differences and and the level of formality appropriate to your situation.
In Australian English, "veggie" is an informal way of saying "vegetable". In certain formal contexts, for example books about health or recipe books, "veggie" is frequently seen. However, in technical contexts (e.g. "is a tomato a fruit or vegetable?", "ginger is technically a vegetable") it is unusual to say "veggie".