it's not just a matter of definition.
if profits form part of a surplus that results from a difference between gains and losses, it behooves us to ask about the substance of that surplus and how/why it arises.
there are two possibilities:
A. the surplus arises in circulation (in which case there is unequal exchange of value between buyers and sellers)
B. the surplus arises in production (in which case we assume that exchange of equivalents is generally the rule)
Looking only at the surface appearance of things it seems as though profits (and thus surplus) are generated in circulation when buyers provide sellers with money. This implies that there is an exchange of non-equivalents between buys and sellers. (I.e. profit could be assumed to arise from sellers "overcharging" relative to the value of a good.) But at the outset this leads us into difficulties if we imagine an economy exclusively dominated by buying and selling which lacks any sphere of production. Such an economy would only circulate goods but not create any. There would be no creation of wealth, nor value, nor surplus. Even in a financial sense, money would simply change hands but the sum would never increase.
Which leads us to profit somehow arising in the sphere of production. Based on surface appearances, the profit might arise by producing more stuff. Profit results from producing more goods which then increase the surplus. But if we assume that there is generally an exchange of equivalents, producing more goods would always result in a no-profit situation unless one could somehow produce goods for free or at reduced cost and then, as the capitalist, pocket the difference. So the question is, where does this difference come from within the sphere of production?
We are then left with:
Machinery / Means of Production
Profit couldn't, as a rule, arise from buying raw materials below value since that violates our general rule of exchange of equivalents. Machinery / Means of Production likewise must be assumed to be bought at their value. This leaves us with labor as our last option. But wouldn't it also violate the rule of exchange of equivalents to say that labor was paid less than its value? Yes, it would. Which is why it's important to understand exactly what is the value of labor and how does it tie in with the production process. If we assume that wages equal the value of labor, then we must draw two conclusions about the value of those wages:
1. They must be sufficient to reproduce that labor to continue production. (i.e. wages must be sufficient to pay for basic necessities and to keep workers from leaving.)
2. Wages can never equal or exceed the total value of goods produced. (Because this would then create a situation in which any profit, let alone surplus, becomes mathematically impossible.)
So now we are left with the conclusion that the combination of raw materials, machinery, and labor forms the basis of the surplus and thus profit. But labor, contrasted with materials and machinery, seems to alone possess a difference between the value it receives and the value it is capable of producing. Thus, Marx concludes that profit arises from this difference. Specifically, Marx drew the analogy to feudalism that serfs worked so many days for themselves and so many days for the feudal lord. Likewise, the wage-laborer works so many hours for himself (to create the value for his wages) and so many hours for the capitalist (to create the value of the surplus.)
This is the explanation for the theory of profit accrual via exploitation.