Bug

In short: A bug is an error in software that causes the program to crash or react differently than expected.

In more detail: The word means “insect”. Origin story: a team once really found an insect (a moth) in the system because of a fault. The anecdote is often attributed to Grace Hopper and the Harvard Mark II computer (1947). The term “bug” for technical faults was already in use earlier.

In Depth

• Types: syntax errors (code not written correctly), runtime errors (crash during operation), logic errors (runs but calculates wrongly) and security holes.
• Causes: typos, wrong assumptions, unforeseen input, interplay of several parts.
• Severity: from a small cosmetic flaw to total failure or data loss.
• Handling: report the error (ticket with steps to reproduce), find and fix it: see debugging.
• Prevention: tests, code reviews and clean input validation.

See also: debugging concept, security in web development