What is a 'bug' in programming?

In programming, a 'bug' refers to a defect or flaw in the code that leads the program to produce incorrect, unexpected, or unintended results. This can manifest in various ways, such as crashes, logical errors, or failure to execute as intended. Bugs can arise from a variety of sources, such as syntax errors, logical errors in algorithms, or unexpected interactions between different parts of the code. Therefore, understanding bugs is critical for programmers, as they must identify and fix these issues to ensure the software functions correctly and reliably.

The other options provided do not align with the definition of a 'bug.' For instance, a code optimization technique aims to improve the efficiency or performance of a program but does not inherently relate to errors in functionality. Similarly, a security flaw specifically addresses vulnerabilities that could be exploited for malicious purposes, while an intentional undocumented feature refers to aspects of the software that may not be officially recognized, but do not constitute a bug in the traditional sense of malfunctioning code.