The term 'Bygone Numbers' refers to the numbers that were used in the past but are no longer in use. The crossword solution for this term is 'Ethers', which is a type of chemical compound that was used in the past but is no longer used today. Ethers are organic compounds that contain an oxygen atom connected to two alkyl or aryl groups. They were once used as anesthetic agents, but have since been replaced by safer alternatives.