How many economic activities are present today?

Economics comes from the Greek word oikonomia, which means a household. The English word means management, as the high school class "Home Economics". Since management always boild down to pinching pennies, the word comes to mean frugality and the total of all spending in some area, measured in the local currency.

Suppose you buy a pencil. Which one? Did you look for a certain color? Was the price important to you? Or did you take the one nearest the cash register? Everybody makes such choices every day, and the total of all of them in an area is called the economy of that area.

So any time you talk about the economy, or economics, or economic activities, you have to stop and be sure everybody knows what you are talking about. That is so important that the big banks and politicians have developed a special language that seems to justify inflation and tax-and-spend policies. It is called "Keynesian".