noun

• (uncountable, also figurative) Increase by natural growth, especially the gradual increase of organic bodies by the internal addition of matter; organic growth; also, the amount of such growth.
  "Warwick was unable to perceive much change in the market-house. […] There might have been a slight accretion of the moss and lichen on the shingled roof."
• (uncountable) (Gradual) increase by an external addition of matter; (countable) an instance of this.
  "Near-synonym: accumulation"
• (uncountable) (Gradual) increase by an external addition of matter; (countable) an instance of this.
• (uncountable, also figurative) Followed by of: external addition of matter to a thing which causes it to grow, especially in amount or size.
  "[W]hile ſome fevv grevv rich by turning Money in their ovvn Banks, there vvas a falſe Appearance of VVealth vvithin, but no Accretion of Riches from abroad."