Learn the Lingo- Plumb vs. Level
The terms “plumb” and “level” sometimes get switched around, but the two words are not interchangeable.
Not to be confused with the fruit, “plumb” refers to a line that is exactly vertical or perpendicular to a horizontal plane. To check if a wall is plumb you can use a tool called a “plumb bob”, which is a weight that has a string attached to it.
Plumb refers to something, a wall in most cases, is straight up and down. The term “level” refers to something being straight from side to side- like a floor, if your floor is not level you may have some problems. You can use a level to check if a surface is even by using a spirit level (sometimes called a bubble level, or just level).
A spirit level is a horizontal plane that has a vial of liquid with an air bubble encased within it. When the level is placed on a surface the bubble of air will indicate whether or not the surface is parallel to the horizontal plane.