An object and/or process that is seemingly unimportant and inactive but becomes crucial to the reaction or process at hand
1. the coco butter in the bath bomb provided the element of fat that was unexpectedly necessary for the foam to occur as it protected the reaction from dissipating.
2. the bodies of water surrounding the city of Ottawa were unexpectedly necessary elements that have influenced the form of the city; all roads have been aligned in parallel or perpendicular to the water.
< English; unexpected meaning unforseen or surprising
< Middle English; necessarie
< Latin; necessaries meaning being essential, indispensable, or requisite