secret (EN)
noun, adjective

noun “secret”

sg. secret, pl. secrets or uncountable
  1. information meant to be kept hidden from others
    After school, let's meet by the old oak tree; I have a secret to tell you.
  2. the essential understanding or method needed to understand or solve something
    The secret to her delicious cookies is a pinch of cinnamon she adds to the dough.

adjective “secret”

secret, non-gradable
  1. not visible or known to others; concealed
    She wore a secret compartment in her ring, where she hid a tiny photograph.