agglomeration

Short definition from Wiktionary

Noun

1. 1
   The act or process of collecting in a mass; a heaping together.
2. 2
   State of being collected in a mass; a mass; cluster.
3. 3
   An extended city area comprising the built-up area of a central city and any suburbs linked by continuous urban area.
4. 4
   A mass of large volcanic fragments bonded under heat.