Whatever could it be

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Across
  1. 2. the act of gathering things together
  2. 8. something we have
  3. 10. nearly; almost; approximately
  4. 11. It's you, this one's free
  5. 12. mathematical symbol
  6. 13. a lack or deficiency of something
  7. 16. series of past events
  8. 17. exists
  9. 18. in honour of; after
Down
  1. 1. Everything
  2. 3. relating to the ideas, art, knowledge, and customs of a society
  3. 4. the one being adressed
  4. 5. Scientific name for a taxon of plants
  5. 6. 359th day
  6. 7. It's me
  7. 9. human organ
  8. 10. a type of logic gate
  9. 14. of or relating to them or themselves
  10. 15. many in building 71