hiit

As nouns the difference between hit and hiit

is that hit is while hiit is (exercise) h'igh '''i'''ntensity]] '''i'''nterval [[training|' t raining; a form of exercise that utilizes short bursts of intense effort, such as sprinting.

In exercise terms the difference between cardio and hiit

is that cardio is cardiovascular exercise while hiit is high intensity interval training; a form of exercise that utilizes short bursts of intense effort, such as sprinting.

As an adjective cardio

is cardiovascular; pertaining to the health of the circulatory and respiratory systems.

As nouns the difference between underline and hiit

is that underline is a line placed underneath a piece of text in order to provide emphasis or (in electronic documents) to indicate that it should be viewed in italics or that it acts as a hyperlink while hiit is (exercise) h'igh '''i'''ntensity]] '''i'''nterval [[training|' t raining; a form of exercise that utilizes short bursts of intense effort, such as sprinting.

As a verb underline

is to draw a line underneath something, especially to add emphasis; to underscore.

As an adjective underline

is passing under a railway line.