Multiplatform docs here: https://kotlinlang.org/docs/reference/multiplatform.html

Example from the docs:

  // Common module
  
  package org.jetbrains.foo
  
  expect class Foo(bar: String) {
      fun frob()
  }
  
  fun main(args: Array<String>) {
      Foo("Hello").frob()
  }

  // JVM module
  
  package org.jetbrains.foo
  
  actual class Foo actual constructor(val bar: String) {
      actual fun frob() {
          println("Frobbing the $bar")
      }
  }

I brought up how I thought the `expect` and `actual` keywords (but primarily the latter) added unneeded verbosity in the Kotlin Slack awhile back and was pretty much shrugged off, despite folks on the Kotlin team admitting that they exist in part to make the IDE's job easier, but weren't actually necessary.

It leaves a sour taste in my mouth, but I suppose it might be because I'm familiar with projects in C/C++ where, if a symbol goes undefined, the compiler/linker is smart enough to tell you that without being hand-held.