Dictionary Meaning of "Modus Ponens" and "Modus Tollens"

Basically, modus ponens is that $P\to Q$ is the mode (or justification), by which affirming $P$ affirms $Q$. $$P\to Q, P\vDash Q$$

Likewise, modus tollens is that $P\to Q$ is the mode by which denying $Q$ denies $P$.$$P\to Q, \neg Q\vDash \neg P$$

By ‘mode’ it means deduction rule, and ‘affirms’ means asserts the truth of some proposition.

They are the abbreviation of the Latin sentences modus ponendo ponens and modus tollendo tollens.

They're the deduction rules which justify the following reasonings, in ordinary language:

• if $A$ implies $B$ is true and if $A$ is true, then $B$ is true (modus ponens),
• if $A$ implies $B$ is true, and if $B$ is false, then $A$ is false (modus tollens).