From DR. BOLI’S UNABRIDGED DICTIONARY.

Organelle (noun).—A small organ. Organelles are either positive or negative; see the illustrations.

Positive organ

A positive organelle, by Pufui PcPifpef, CC BY-SA 4.0, via Wikimedia Commons.

Negative Organ

A negative organelle, by Anathea Utley, CC BY 2.0, via Wikimedia Commons.