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Define a method with def:

def square(n: Int): Int = n * n

def greet(name: String, greeting: String = "Hello"): String =
  s"$greeting, $name!"

The last expression is the return value. Explicit return exists but is rarely used (and discouraged).

Multi-statement bodies use braces:

def factorial(n: Int): Int = {
  var result = 1
  for (i <- 1 to n) result *= i
  result
}

Function values (anonymous functions) use =>:

val double = (n: Int) => n * 2
val add = (a: Int, b: Int) => a + b

double(5)         // 10
add(3, 4)         // 7

Difference between def (method) and val ... => (function value): mostly transparent — in most contexts they're interchangeable. Functions are first-class values.