Consider a User model with a validation for the name attribute like this:

class User < ApplicationRecord
  validates :name, presence: true

The key for the error message in this case is :blank. Active Record will look up this key in the namespaces:

Source: Rails Internationalization (I18n) API - Ruby on Rails Guides

This link contains a table of all the message names for use with config/locales/en.yml (or any other locale). The names are somewhat arbitrary, so it’s handy to have this for reference.