Official acts and guides shed light on what kinds of income there can be and distinguish between taxable and non-taxable income. However, I could not find any definition or even a hint of what is income and what is not income, even in the Income Tax Regulations. As far as I understand, the Regulations only list "Inclusions" (the amounts that are definitely income) and "Deductions" (the amounts that are income but can be deducted).
For example, if I return an item to the store, I receive a refund. Common sense tells me that it is not income, but is there any document that says so?
Other examples may include giving and collecting a loan (without interest), receiving a monetary compensation from a stranger who spilled coffee on your laptop and similar.
My question is specific to Canada, but answers for other jurisdictions are welcome too.