You can use the adjective supplemental to describe something extra. You could volunteer to donate your collection of markers and colored pencils to a local school that needs supplemental art supplies.

Something that's supplemental is added when there's a lack or deficiency. You might feed the runt from a litter of puppies supplemental milk because he can't get enough from his mother. Teachers are famous for assigning supplemental reading to round out the information in a textbook. Supplemental comes from the Latin supplementum, "added to supply a deficiency."

Definitions of supplemental

adj functioning in a supporting capacity

auxiliary, subsidiary, supplementary
being of second rank or importance or value; not direct or immediate

adj added to complete or make up a deficiency

characterized or produced by addition

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