A 'bug' in the context of software development refers to a flaw or error in the code that leads to unintended behavior or outcomes. Bugs can stem from various causes, such as logical errors, syntax mistakes, or unforeseen interactions between components. Their presence can disrupt functionality, degrade user experience, or cause crashes. Identifying and fixing bugs is a critical part of the software development lifecycle, often requiring rigorous testing and debugging processes to ensure the final product is reliable and performs as intended.