A BASIC program is a series of lines containing instructions for the BASIC
  compiler.   A  BASIC  program  line can contain a line number, one or more
  statements, an optional comment field, and a line terminator.

  LINE is also a special immediate mode variable that  allows  the  user  to
  determine  what statement was executing if a program was interrupted.  For
  example, if a CTRL/C is typed when a program was executing line  100,  the
  immediate mode statement PRINT LINE would display 100.