"Three things: logic is said to be formal (or “topic-neutral”)
(1) in the sense that it provides constitutive norms for thought as such,
(2) in the sense that it is indifferent to the particular identities of objects, and
(3) in the sense that it abstracts entirely from the semantic content of thought."

WHAT DOES IT MEAN TO SAY THAT LOGIC IS FORMAL? John Gordon MacFarlane, 2000

"Formal logic is the study of inference with purely formal content. An inference possesses a purely formal content if it can be expressed as a particular application of a wholly abstract rule, that is, a rule that is not about any particular thing or property." Wikipedia