Algae (noun)

1. A large group of diverse, mainly aquatic, simple, plant-like organisms, ranging from single-celled forms to large seaweeds.
2. Microscopic plants that are found in water and are capable of photosynthesis.

Origin:

Late 18th century: modern latin, singular alga, from latin, "seaweed", from greek algos "pain" (because of the slimy texture of seaweed).

Examples:

1. The lake had a thick growth of algae.
2. Algae is an important source of oxygen in the ocean.
3. Algae can be used as a source of biofuel.
4. Algae is used as a food supplement for humans and animals.
5. Algae is also used in cosmetic products.