a. a method or device for reducing harmful emissions from internal-combustion engines before they are released into the atmosphere, for example a filter or a catalytic converter
⇒ This particular filter, introduced in 2007, will be the only aftertreatment required
for heavy-duty engines in 2010.
b. (as modifier)
⇒ an aftertreatment system
3. textiles a chemical treatment that is applied to a fabric immediately after it has been dyed, in order to make it more colour-fast
Collins English Dictionary. Copyright © HarperCollins Publishers