An Algae is a simple rootless plant that grow in sunlit waters in proportion to the amount of available nutrients.

They can affect water quality adversely by lowering the dissolved oxygen in the water. They are food for fish and small aquatic animals.

Other /More definition:
An Algae is a chlorophyll-containing plant ranging from one to many Cells in size, that lives in fresh or salt water.

see also:
"Algae" is in the UNSPSC Code "85141602"
Algae or sea weed medical cures

"Algae" is in the NAICS Code "11199"
All Other Crop Farming
Algae farming

Other /More definition:
An Algae is simple one-celled or many celled plants capable of photosynthesis, usually aquatic.