The root word is "differ". "difference" = "what differs" or "the act of differing" Consider also: "prefer" => "preference" = "what is preferred" or "the act of preferring" "refer" => "reference" = "what is referred to" or "the act of referring" Also consider: "confer" => "conference" = "an event where people confer with each other" "infer" => "inference" = "what is inferred" or "the act of inferring"

It's the verb to 'differ'.

Difference is the noun, from 'to differ' the verb. To differ means the same as 'to be different from'. For example, "the two sisters differ [from each other] in height".

Etymology: from the Latin verb DIFFERRE: to separate, to send away, to remove.