Although one of the least common definitions, a "line" can also mean 1 fluid oz of codeinecough syrup. For example, some people who sell the stuff will sell it by the "line" for anywhere from like 30-100 dollars.
Yo Cleophis, how much is that Activis goin for per line?
Powdered substance (usually cocaine) that has been separated into multiple lines of powder usually by using a credit card or razor. These lines of powder are then sucked up through the nose usually using a rolled up dollar bill or an empty pen tube.