Backyard-Back-Yard | Commonly Confused Words

Backyard

/ˌbækˈjɑːd/

The area behind a house, typically used for recreation or gardening.

This word has no antonyms

/bæk jɑrd/

An area at the back of a house, typically covered with grass and used for leisure activities.

Backyard is usually a single word when used as a noun to describe the area at the back of a house.
When describing the concept of a yard located at the back, back yard can be used, especially when emphasizing the spatial arrangement.
In casual conversation or writing, backyard is often preferred as it flows more naturally.