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In website development  a ‘bug’ is a mistake or unwanted piece of code that keps a website or program from working like it should. This could be any part of the code, whether front-end code (HTML, CSS) or back-end code (such as PHP, Javascript, Ruby, etc.). More basically, we call something a bug when it’s not working as expected or designed.

The process of finding and fixing bugs is termed ‘debugging’ and often uses formal techniques or tools to pinpoint bugs. A website that contains many bugs and/or bugs that seriously interfere with the functionality of the site is said to be “buggy” or “defective”.  Generally bugs can be categorized into different priority levels, ‘critical’, ‘high’, ‘low’, or ‘deferred’, which helps developers prioritize which bugs need to be tackled first.

Bugs can trigger errors that have a ripple effect causing functionality issues with other seemingly unrelated areas of the website.