adverb (or adjective)
1.
a. : on a level with the eye of a spectator — used of the arrangement of a picture in an exhibition
b. : in full view and at hazard
put his future on the line when he backed this policy
2. : on the border between two categories of a classification
3. : immediately : without delay
paid cash on the line
4. : into prostitution
she lost her job and went on the line