A pun is a play on words using two words that have different meanings but sound alike (Dr. Wheeler, "Literary Terms and Definitions"). One pun Duke Orsino speaks in Twelfth Night can actually be found in the very first scene. Orsino's servant Curio suggests Orsino go hunting to try and distract him from pining over Olivia. Specifically, Curio suggests that Orsino go hunt "[t]he hart," which is another word for deer (I.i.18). However, in his next lines, Orsino makes a pun out of the word hart by interpreting it to mean heart, as we see in his line, "Why, so I do, the noblest that I have," which is to say that he already does engage in hunting the "hart," the "noblest" heart that he has (19). He continues his pun further to explain that the moment he laid eyes on Olivia was the moment that his heart became hunted like a deer, hunted his own desires that he likens to cruel hunting dogs.
This very same speech in which Orsino puns hart and heart is actually also an example of an allusion. An allusion is when one author refers to another work of literature by referring to either a "person, place, event, or another passage" (Dr. Wheeler). Any references to Greek or Roman mythology serve as allusions because both the Greeks and Romans recorded their mythologies in writing. Orsino's speech in which he likens himself to a deer being hunted is actually an allusion to a Greek myth. The myth is the story of the hunter Actaeon who once while on a hunt caught sight of the goddess Artemis bathing in the woods. As punishment, she turned him into a deer to be hunted and torn asunder by fifty hounds (Shakespeare Online). In Orsino's speech, Olivia is being represented by Artemis, and Orsino is representing himself as the hunted hunter Actaeon, making it a perfect allusion to Greek mythology.