accretion

noun

• growth or increase by the gradual accumulation of additional layers or matter:the accretion of sediments in coastal mangroves figurativethe growing accretion of central government authority
• [count noun] a thing formed or added by gradual growth or increase:the city has a historic core surrounded by recent accretions
• Astronomy the coming together and cohesion of matter under the influence of gravitation to form larger bodies: during the later stages of accretion the outer 100 km or so of the Moon melted