noun

• (uncountable) The act of one that understands or comprehends; the mental process of discernment of meaning.
  "There are certain things that defy human understanding."
• (countable) Reason or intelligence; ability to grasp the full meaning of knowledge; ability to infer.
  "She has a solid understanding of particle physics."
• (countable) Opinion, judgement, or outlook.
  "It is the goneness of the Holocaust that produces the simultaneous profusion of discourses and understandings; the goneness is what opens up, what spurs, what unleashes the perpetual desire to do, to make, to rethink the Holocaust."
• (countable) An agreement of minds; harmony; something mutually understood or agreed upon.
  "I thought we had an understanding - you do the dishes, and I throw the trash."

adj

• (dated) Knowing; skilful.