What term describes the act of describing something as less than it is?

The term that describes the act of describing something as less than it is is "understatement." This rhetorical device is used to downplay a situation or a characteristic, often for ironic or humorous effect. For example, saying "It's just a scratch" when referring to a large dent in a car exemplifies understatement because it minimizes the severity of the damage.

In the context of rhetorical devices, understatement can create an impact by drawing attention to the reality of a situation while using a softer or more reserved language. It often invites the listener or reader to read between the lines and perceive the situation in a more profound manner.

Similes, on the other hand, are comparisons that use "like" or "as" to draw similarities between two different things, while hyperbole involves exaggerated statements or claims not meant to be taken literally. Personification attributes human qualities to non-human entities but does not pertain to the act of diminishing the significance or size of something.