Correct spelling, explanation: reservoir derives from a French form reserver, which had the meaning of to keep and to reserve. Reservoir entered English in the 17th century, but its pronunciation still connotes French origin. In modern English reservoir is a noun.

Definition of reservoir:
1. noun – a type of lake, which can be either man-made or natural,
The reservoir has been contaminated with some unknown substance, which can be a threat to the whole region.
2. noun – when there is a large amount of something,
It’s an amazing thing that you have such a vast reservoir of love for unwanted children.

Expressions with reservoir:
Some commonly used expressions with reservoir are: storage reservoir, water reservoir, and reservoir dam.
The construction you can see on your left is a storage reservoir, which was built over 200 years ago, but it has still been used.
It’s a poor and extremely dry region, which can be saved only by building a new water reservoir.
The amount of water is controlled by a recently built reservoir dam, which protects the city from flooding.

Incorrect spelling, explanation: when reservoir is pronounced, the middle -r letter isn’t heard in speech. As a result, many English users think that the same happens in writing and they spell the word as resevoir, which is wrong. The correct spelling is with the -r letter in the middle, which gives reservoir.