The meaning of words is in how they are actually used, not by what the dictionary says they mean. After all, that's how the dictionaries come up with the definitions in the first place -- by looking at all the ways the words have appeared in writing, and by coming up with a set of common descriptions.
So the easiest way to explore the meaning of something like "altruism" is to read it in action. Let's look at some quotes:
Every man must decide whether he will walk in the light of creative altruism or in the darkness of destructive selfishness. - Martin Luther King, Jr.
Clearly we can assume, from this, quote, that the word has a positive connotation that contrasts with selfishness. So altruism must be somewhat synonymous with selflessness. Not much to go on, but it's a start.
I don't deny the importance of genetics. However, the fact that I might be altruistic isn't because I have a gene for altruism; the fact that I do something for my children at some cost to myself comes from a history that has operated on me. - B. F. Skinner
A different angle. Skinner (a renowned psychologist and behaviorist) says doing "something for my children at some cost to myself" is an example of altruistic behavior -- that altruism necessarily includes some measure of sacrifice.
Countries that intervene militarily rarely do so out of pure altruism. - Samantha Power
Wars are never fought for altruistic reasons. - Arundhati Roy
Again, altruism in both these quotes seems to mean, "the desire to do good at some cost to oneself". Power says that military intervention, no matter how well-meaning, is often done out of at least some measure of self-interest, and therefore altruism must imply the complete lack of self-interest and with good intentions.
Roy, on the other hand, suggests that violent conflict, even with good intent, can never be altruistic. Therefore her definition must include the admonition to do no harm.
And so on. We could continue to refine the definition through various other excerpts from written sources, but let me wrap up with this quote from the noted economist Adam Smith, whose "Wealth of Nations" is often said to be the first work of modern economics. But even he recognized that people often do things for reasons other than the purely monetary:
How selfish soever man may be supposed, there are evidently some principles in his nature, which interest him in the fortune of others, and render their happiness necessary to him, though he derives nothing from it, except the pleasure of seeing it.
To paraphrase: Smith says that there is the fundamental desire in humans to enhance the well-being of others, even if the giver gets nothing from it other than personal pleasure -- which is, I think, as good a definition of altruism as any.