Is a "selfless" act defined by its intentions or its consequences?

I will hear people often claim that there is no such thing as a selfless act. People will cite instances like donating to charity that results in a feeling of satisfaction and pleasure in you because you donated, thus proving that it benefits you and is not “selfless”.

But, to me, a selfless act is one that has selfless intentions. So, if you donate to charity because you want to help others and you don’t care about how it makes you feel, then the act is selfless whether or not you “feel good” as a result. It’s about the intention, not the consequences. Your action was motivated by a selfless intention, thus the action is selfless, even if it benefits you as a result.

However, I know many do not agree with this. Is it about intentions or consequences? The biggest problem for me is that you can never truly “prove” what your intentions were…