A function in JavaScript is a block of code that can be called by name and executed as many times as needed. A function can accept parameters (inputs) and can return a value. To define a function in JavaScript, you use the "function" keyword, followed by the function name, and the function parameters in parentheses. The code that the function executes is enclosed in curly braces {}.

Example:

function myFunction(param1, param2) {
  // code to be executed
}