A gift occurs when someone has given a material given to someone else who liked the received given.
Gift is commonly used for meaning the action of giving and not asking for anything material in return (for that given) (there's not a material quid pro quo).
A gift provoques an outcome between the involved parties. The giver gets released, the receiver gets improved. Gift economy is used simultaneously along with the sharing economy, sharing practice, etc. for evolutionary models between large groups of people.
Gift is also used for gift to world, gift to the nature that may not have the capacity (or mistake) of liking the gift.
Altruism (also sharing) is a gift including human values in the act of gift. Both gift and altruism have its defining limitations: you are altruist when someone has taken advantatges on you, you could better have been solidary instead.
Gift is used in a part of the barter definition gift is the additional value someone gives to someone else for assuring himself the equivalence is reached because we always tend to value (or suspect) that the value of the things we give in the barter are more valueable than our barter mate's ones (ego-?).