Find your own definition
The hard answer is that there is not really a definition of economics that can satisfy all economists or a definition that captures the study of the economy as a whole.
However, the important thing for you as an economics student is to try and find a definition of economics that best suits you. In your research on the different definitions of economics, you will come to understand what a wide range of economists, past and present, think in their field. What’s more, you too will gain a broader appreciation of the subject as a whole and develop opinions about your interests.