What is a 'lambda' function in Python?

A 'lambda' function in Python is best described as an anonymous function expressed as a single line. It allows you to create small, throwaway functions without needing to formally define them using the standard def syntax. This is particularly useful in situations where you require a function for a short period, such as when using higher-order functions like map(), filter(), or sorted().

Lambda functions can take any number of arguments but are limited to a single expression. The expression's result is automatically returned, making them concise and effective for simple functionalities. Because they are anonymous, lambda functions do not have a name, which distinguishes them from regular functions defined with def.

While other options touch on different aspects of functions, they do not accurately characterize what a lambda function is within the context of Python programming. Understanding the nature of lambda functions can be essential for efficient coding and functional programming techniques in Python.