local

^^^ input is for things like values you want to provide at "terraform apply" runtime (like sensative secrests you don't want checked into source control), output is for created resource info you wan to highlight at end of "terrafrom apply" or pass to child module (like elastic ip created), local is just used within the *.tf configs so you are not repeating code (and that things are easy to change in just one place)

^^^^ local variables are declared and defined within "locals" blocks in the *.tf files, and is just a map of values, e.g.

locals {
  default_name_prefix = "${var.project_name}-web"
  name_prefix         = "something"
}