What one party adds to a common thing. A contribution is named from the giving side and measured on the common one.
Latin contributio (a bringing together, contribution), from contribuere (to bring together, add), from com- (together) + tribuere (to grant, allot); what one party adds to a thing held in common.
Latin tribuere is to allot, and a contribution was first a share assigned toward a common purse, not a personal gift kept track of for its own sake. What it is worth is decided by the pool it joins, never by the hand that added it.