noun Date: 15th century a line (as of cord) that has at one end a weight (as a plumb bob) and is used especially to determine verticality, a line directed to the center of gravity of the earth ; a vertical line
PLUMB LINE
Meaning of PLUMB LINE in English
Merriam Webster. Explanatory English dictionary Merriam Webster. Толковый словарь английского языка Мерриам-Уэбстер. 2012