To flatten means to make something flat or to remove its thickness.
Imagine you have a big box of clothes that's all squished up and hard to carry. You might need to flatten it out so it's flat and easy to fold up and put away. This is what happens when you take the air out of an inflatable mattress too - it starts to flatten down. Sometimes we use machines to help us flatten things, like a big ironing press for thick blankets or clothes.
How common is "flatten"?
Word flatten is considered uncommon in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
verb
To make something flat or flatter.
Example: As there was a lot of damage, we chose the heavy roller to flatten the pitch.
To press one's body tightly against a surface, such as a wall or floor, especially in order to avoid being seen or harmed.
To knock down or lay low.
Example: The prize fighter quickly flattened his challenger.
To become flat or flatter; to plateau.
Example: Prices have flattened out.
To be knocked down or laid low.
To lower by a semitone.
To make vapid or insipid; to render stale.
To reduce (a data structure) to one that has fewer dimensions, e.g. a 2×2 array into a list of four elements.