When a program is running, or executing, it is said to be in runtime. The term is mostly used by software developers to specify when errors in a program occur. A "runtime error" is an error that happens while the program is executing. For example, if a program told you that 2 + 2 was 5000, that would be a runtime error. A memory leak, where the program sucks up excessive amounts of system memory is also a runtime error. The other major type of program error is a compile-time error, where the application will not even compile into an executable program. Examples of compile-time errors are syntax errors in the program code and file linking errors. Runtime is a good term to know, but you most likely won't hear it used very often unless you like to hang out with computer nerds.