An assertion is a command of the form assert(boolean_expression) that prints an error and exits the program if the boolean expression is false.

An assertion is a statement in programming that lets the programmer test if a certain condition is met and throw an error if it isn't. It is used for debugging purposes.

In the form assert(boolean_expression), the boolean_expression is the condition that we are testing. If this condition is true, then nothing happens and the program continues to execute. However, if the condition is false, then an AssertionError exception is raised and the program stops running.

This is useful for catching bugs in your code. For example, if you have a function that is supposed to always return a positive number and you want to make sure that it is doing so, you can write assert(function() > 0). If at any point the function returns a non-positive number, the assertion will fail, alerting you to the problem.