There were many definition on economics.
some of them are:
"Science of wealth", the problem with this was that, people without much wealth would be considered poor and the problem between rich and poor will start for no reason, and this people are know as classical thinkers.
Then came "Science of welfare", welfare is both quantitative and qualitative in nature, Consumption of goods and services, increase in per capital income etc are quantitative aspects of welfare. Living in peace, enjoying leisure, acquiring knowledge etc are qualitative aspects of welfare. But the problem was, economics was said to be concerned with the quantitative welfare only because it can be measured in terms of money.
Then came "Science of scarcity and choice", after that "Science of growth and development", and finally "Science of sustainable development"
In the end it just came to be known as "The branch of knowledge concerned with the production, consumption, and transfer of wealth."
Basic thing you need to know