Why are you using these terms interchangeably? They don't have interchangeable meanings. They don't even refer to one another.
ALTRUISM
1. Unselfish concern for the welfare of others; selflessness.
2. Zoology. Instinctive cooperative behavior that is detrimental to the individual but contributes to the survival of the species.
CHARITY
1. Provision of help or relief to the poor; almsgiving.
2. Something given to help the needy; alms.
3. An institution, organization, or fund established to help the needy.
4. Benevolence or generosity toward others or toward humanity.
5. Indulgence or forbearance in judging others. See Synonyms at mercy.
6. often Charity Christianity. The theological virtue defined as love directed first toward God but also toward oneself and one's neighbors as objects of God's love.
--According to Dictionary.com
Altruism is not selective. It is indescriminate. It does not consider the need or plenty of the recipients. It is concern for others potentially to the cost of one's self.
Charity is an act which discriminates to the benefit of the needy. It is also a conscious attitude of benevolence to others. It presumes continuity of the self to maintain it. Even Christian charity, which seems quite altruistic, requires the giver to think of one's self as well.
A gift that is given without discrimination or thought is not charity. Nor is the choice to die so that others may live. Neither set of definitions bestow inherent "goodness" on the act or the giver. I would say goodness about the act or the giver depends upon the context surrounding the event.
I will be on the road this week anbd will respond to all of you when I come back.
Agreed. Nor is it altruism. I use both terms interchangeably because in the given context they are. Both refer to an act of giving away something of value without an expectation of a material reward of similar value. If you think I used one term where the other would be the only approriate, please show me.