by extension

-- also, by extension, any one in the homologous series of paraffine alcohols of which methyl alcohol is the type.

drawn together or squeezed physically or by extension psychologically

also, by extension, to other elements sometimes found in the same connection

The art or practice of curing the deformities of children, or, by extension, any deformities of the human body.

hence, by extension, any substance, whether solid, liquid, or gaseous, of the same chemical series