In computer programming terminology, an error or defect in a program that causes it to behave unexpectedly is commonly called what?

Introduction / Context:
Software rarely works perfectly the first time it is written. Programming errors are very common, and there is a widely used informal term for such defects in code. Understanding this term is important because developers, testers and technical documentation all routinely refer to it when discussing problems in software.

Given Data / Assumptions:

• The question asks for the common term used for an error in a program.
• Options include power failure, crash, virus, bug and timeout.
• We assume general software development and troubleshooting contexts.

Concept / Approach:
A bug is the informal but widely accepted term for a fault, flaw or error in a computer program that leads to incorrect or unexpected behaviour. Power failure refers to a loss of electrical power, crash describes a program or system that stops working, virus is a type of malicious software and timeout is a situation where a process takes too long. None of these directly name the defect in the program code itself in the way that bug does.

Step-by-Step Solution:
Step 1: Focus on the wording an error in a program, which points to a problem within the software code. Step 2: In software engineering, programmers and testers commonly say they are fixing bugs, meaning they are correcting errors in the code. Step 3: A power failure is external to the program and occurs when the computer loses electricity; it is not a programming error. Step 4: A crash describes the result of a serious problem where a program stops running, which can be caused by a bug but is not the name of the error itself. Step 5: A virus is malicious software that can infect programs or systems, not an ordinary programming mistake. Step 6: A timeout occurs when an operation takes longer than expected and is often a symptom rather than the root cause of a programming defect. Step 7: The simplest and most precise term for an error in a program is bug, so that is the correct answer.

Verification / Alternative check:
Software development literature and defect tracking tools consistently use the word bug to describe issues in programs. Historical anecdotes attribute the term to early computing, including a famous story about a moth causing a hardware problem, although the word was used more broadly for technical faults even before that. Issue tracking systems, release notes and support forums all talk about bug reports and bug fixes, reinforcing that bug is the standard term.

Why Other Options Are Wrong:
Power failure is a hardware or infrastructure problem and can affect any running program, even if the code is perfect. A crash is an outcome, not the defect itself. A virus is a deliberate malicious program, not an accidental programmer mistake. Timeout refers to a condition where a response does not arrive in time, but it may or may not be caused by a code error. These terms describe conditions or external events, not the internal error in a program that the question is targeting.

Common Pitfalls:
Some learners may confuse bugs with viruses because both cause programs to misbehave. To avoid this confusion, remember that a bug is an accidental mistake by the programmer, while a virus is intentionally created malicious code. Anti virus tools remove viruses; debuggers and testing fix bugs. Keeping these concepts separate makes questions about software errors easier to answer.

Final Answer:
An error or defect in a program is commonly called a bug.