Correct spelling, explanation: reservoir appeared in English in the middle of the 17th century. As the pronunciation suggests reservoir has a French origin as it comes from the French word reserver, which meant to reserve or to keep. Today, reservoir functions as a noun.

Definition of reservoir:
1. noun – usually refers to a natural or artificial lake,
A man-made reservoir, which is not far away from here, is the main source of drinking water for the whole city.
2. noun – a large supply of something,
The Internet is a huge reservoir of information, but it is often difficult to decide which content is truly reliable.

Collocations with reservoir:
A few common collocations with reservoir include: natural reservoir, artificial reservoir, and underground reservoir.
There is a natural reservoir hidden below the surface of this massive waterfall.
This artificial reservoir has saved the region from a devastating drought.
This underground reservoir has been discovered by chance during the construction works.

Incorrect spelling, explanation: a common spelling mistake in reservoir is to write it without the -r letter in the middle of the word. It gives an incorrect form resevoir. The mistake is probably the consequence of the pronunciation as the middle -r letter is silent, but it appears in the spelling so the only correct version is reservoir.