Line of code
The definition of a line of code, though basic to many Software Engineering metrics, is ambiguous. What a "line of code" means will certainly vary across languages, but it also varies for a given language. But when using a standard layout, much of this ambiguousness goes away. Also comments are not code.
In the C language, for example, a line of code might be (1) a statement terminated by a newline, (2) a statement terminated by a semicolon, or (3) any line in the program terminated by a newline (comments included).
for (i=0; i<100; ++i) {printf("hello");} /* How many lines of code is this? */
Keeping these limitations in mind, the KLOC (thousand lines of code) is sometimes used as a unit for measuring a program's size or complexity. Defects per KLOC is sometimes used when measuring a program's quality.