VisiCalc (for "visible calculator") was the first spreadsheet computer program for personal computers, originally released for the Apple II. It is often considered the application that turned the microcomputer from a hobby for computer enthusiasts into a serious business tool, prompting IBM to introduce the IBM PC two years later. Visicalc is considered the Apple II's killer app. It sold over 700,000 copies in six years, and as many as 1 million copies over its history.