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Functions that take or return other functions are higher-order. Scala's standard library leans heavily on them.

def apply3Times(f: Int => Int, x: Int): Int = f(f(f(x)))

val doubled = apply3Times(_ * 2, 1)   // 8

Underscore syntax _ * 2 is shorthand for (x: Int) => x * 2 — each _ becomes a parameter in order.

Classic higher-order on collections:

val nums = List(1, 2, 3, 4, 5)
nums.map(_ * 2)                  // List(2, 4, 6, 8, 10)
nums.filter(_ % 2 == 0)          // List(2, 4)
nums.reduce(_ + _)               // 15
nums.fold(100)(_ + _)            // 115
nums.forall(_ > 0)               // true
nums.exists(_ > 4)               // true

This is where Scala feels different from Java — write composable transformations instead of explicit loops.