Very simply, “diction” refers to the word choice of a text – the fact that some words were chosen to convey a given concept or image, and others weren’t. It’s an easy one to understand and remember, since it’s related to “dictionary,” but not necessarily all that easy to use to make a compelling point within an argument. On a basic level, because poems are purposive artifacts (they don’t happen by accident), every word was chosen. So when we talk about diction, what we’re really doing is pinpointing words that stand out for us because they seem to have been chosen for a particular reason. That means you already have to have a good idea of what the poem was about before you start thinking about word choice, and thus diction should probably not be the first thing you focus on. Before you do that, paraphrase the poem to yourself: figure out who is speaking, where and when they are speaking from, what the situation is, why they are speaking, and what they are saying. Then, take into account whether some of the language may be figurative, and the extent to which the form and meter of the poem are conditioning the content (ideas, images, rhetorical appeal) of the poem, or giving you a different idea of the speaker. For example, consider Siegfried Sassoon’s “Repression of War Experience,” a poem spoken by a trench soldier from World War I suffering from PTSD, recuperating with difficulty back home in England. The content when considered alone can make us think that the speaker is going through a nervous breakdown while he is actually talking. HOWEVA, the form of the poem gives the lie to that idea, since the poem is mostly written in iambic pentameter, a stately rhythm that indicates the poem has been revised many times and is not at all the raw record of an unstable mind.
Thinking about diction is an avenue toward knowing more about the speaker, but not necessarily the only or the best way. To stay with Sassoon for a moment longer, you might have noted that the speaker of this poem uses language that WWI British soldiers might have been expected to use: “jolly,” “glory,” and “bloody,” but then again he also tells us that there’s a war on in France, and that he’s “summering safe at home,” so even a fairly superficial reading can still get you in touch with the speaker and his setting. What diction allows you to do is to be much more precise with the ways you’re identifying that speaker. Instead of describing, you can point to specific moments in the text, and then even if I completely disagree with your characterization, I will have to come up with counterexplanations for why those words are present. It’s a good way to back yourself up when you’re arguing about poetry.
Having said that, there will be some texts that can only be interpreted well if you are paying close scrutiny to diction. T. S. Eliot’s poem “Gerontion” (a dramatic monologue) is a great example of this phenomenon, something akin to slipping in a vital clue surreptitiously that, if you find it, will solve a mystery. That poem looks/sounds like this (check the transcript for what the lines look like):
Here I am, an old man in a dry month,
Being read to by a boy, waiting for rain.
I was neither at the hot gates
Nor fought in the warm rain
Nor knee deep in the salt marsh, heaving a cutlass,
Bitten by flies, fought.
My house is a decayed house,
And the jew squats on the window sill, the owner,
Spawned in some estaminet of Antwerp,
Blistered in Brussels, patched and peeled in London.
The goat coughs at night in the field overhead;
Rocks, moss, stonecrop, iron, merds.
The woman keeps the kitchen, makes tea,
Sneezes at evening, poking the peevish gutter.
I an old man,
A dull head among windy spaces.
[it goes on for another 58 lines]
“Gerontion” is a word that literally means “old man” or “the aged” in ancient Greek. But it also refers to one of the elders of Sparta (who had to be pretty old to qualify for leadership), so there is the possibility that the title’s diction contains an ambiguity: someone who is wheezy, dusty, and bitter, and who feels irrelevant because left out of life in favor of the young; also, a politician. If you consider “hot gates” in line three in light of the Greek in the title, those of you who have seen the film 300 should recognize the reference to the battle of Thermopylae (which translates literally as “hot gates”) at which the 300 Spartans led by Leonidas sacrificed themselves to save Greece from the Persian invasion. HOWEVA, this old man did not sacrifice himself, and he seems not only to be bitter but a raging anti-Semite (he doesn’t even bother to capitalize “jew” as he reflexively hates on his landlord). And we’re not in Greece, but in England just after World War 1.
This is crucial. You can account for the speaker’s knowledge of Greek because wealthy young men of good class were taught not English literature, but classics in private school (Greek and Latin). But you can’t account for his use of “merd” (French for poop), or more to the point, his knowledge of what an “estaminet” is, without the war. An estaminet is a French term for a combination tavern and brothel that was frequented by British soldiers, all of whom were stationed in trenches in Belgium during the war (hence, too, the references to the Belgian cities Antwerp and Brussels). So unless you are attending to the word choice of this passage, you might drastically mistake the setting and time period of the text, and miss the fact that it’s making, at the time of its publication in 1920, extremely topical statements about the effects of World War One, and probably criticizing the old, bitter, clueless politicians who sent so many young men to their deaths.