1. A script file containing the startup instructions for a program (an application or even the operating system). The file, to be executed automatically when the operating system is started, contains a list of instructions (commands or other scripts) to run. From I-gloss

2. Script file containing startup instructions for an application program (or an entire operating system), usually a text file containing commands of the sort that might have been invoked manually once the system was running but are to be executed automatically each time the system starts up. From Linux Guide @FirstLinux