Night is clearly defined by the Sun setting over the horizon and the following rising. The Sun's passage over the meridian is the astronomical midnight.

It’s not always clear. For instance pilots can log night hours once it’s civil twilight, but there’s also nautical twilight and then astronomical twilight, all defined by the number of degrees below the horizon the sun is. What we consider “night” is pretty arbitrary.

The sun passing the horizon is affected by your vertical position. Someone in the lobby of a skyscraper has a different sunset time than someone near the top.