This is a proposal, not decided upon or in effect. Feel free to comment or create you own if you have thoughts.

Definition: Supplying your own skills or resources to someone without expecting anything in return.

It does not mean that there are no conditions whatsoever. In every sharing scenario, there are certain implicit conditions. For example, that no emergency occurs in which the giver desperately needs what he promised to give. A common condition (although this is negotiable on an individual basis) is that the receiver of an item has to pick it up from the giver.

Unconditional sharing also does not mean that the receiver is forbidden to reciprocate. Reciprocity is a social norm and can be realized directly (like voluntarily providing a token of gratitude) or indirectly by supplying to the community whatever else one may have to share (generalized reciprocity) or simply not at all. It's up to the receiver if and how he/she wants to reciprocate.

Unconditional in this sense means that no monetary or other compensation should be offered or demanded as a precondition of the act of sharing. It is thus to be seen in contrast to trade and exchange.

References:

Alvin Gouldner (1960). The Norm of Reciprocity: a Preliminary Statement. American Sociological Review, Vol. 25, No. 2, pp. 161-178.

Hornby, A. S. (2000). Share - Give some of yours. Oxford Advanced Learner’s Dictionary (6 Rev ed.). Oxford University Press, p. 1177.

Yamagishi, T., & Kiyonari, T. (2000). The Group as the Container of Generalized Reciprocity. Social Psychology Quarterly, 63(2), pp. 116–132.

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