Two lines are all Herrick needs to pin down one of the slipperiest words in the language, and the surprise is that he does it without really defining anything. A definition normally fixes a word in place: this is what it is, this is what it is not. Herrick answers the question "What is beauty?" with a picture of light. Beauty, he says, is "no other thing" than a beam, and in the same breath he takes the "thing" away, because a beam of light is not an object you can hold. It happens, and then it passes.
The central idea sits in the second line, where the beam is "Flash'd out between the middle and extreme." The verb carries a lot. "Flash'd" is sudden, almost abrupt, and it suggests that beauty is not a steady property of an object but an event, something that sparks into being. It sparks "between", in a gap or a relationship. The poem never says what the middle and the extreme are, and that openness is part of its appeal. Judging by the wording, one reasonable reading is that beauty comes from balance, from the right proportion between moderation and excess, or between the parts of a face or a form. Another is that it appears where the ordinary meets the startling. On either reading, beauty lives in the relation between things rather than in any one of them.
The construction supports this. The poem is a single rhymed couplet, and each line has ten syllables in a roughly iambic pentameter pattern. The first line twists ordinary word order: we would normally say "Beauty is no other thing than a beam," but Herrick writes "Beauty no other thing is, than a beam." The inversion gives the line the weight of a ruling handed down, and it puts the stress on "no other thing", a flat refusal of any competing answer. Then the sentence runs straight over the line break with no punctuation to stop it. The word "beam" ends the first line, and "Flash'd out" begins the second, so the line ending acts out the flash. The beam shoots out of one line into the next.
Sound does quiet work too. The long *e* of "Beauty", "beam", "between" and "extreme" runs through the couplet like a ray passing through a room, and the *b* of "Beauty", "beam" and "between" ties the key ideas together. "Beam" and "extreme" rhyme, and the rhyme links the light with the idea of extremity. The couplet ends on a closed, humming *m*, as if the line had been struck and was still ringing. It is also hard to miss that "middle" sits roughly in the centre of the second line, while "extreme" is pushed to the very end. The line is arranged the way it describes.
There is a possible second meaning in "beam". Besides a shaft of light, the word can mean a timber that spans the space between two supports. The poem never says so, so this is only a suggestion, but it would fit the idea of beauty as something that bridges a gap between two points.
The title matters as well. "Definition" promises logic, a tidy statement of what beauty *is*. What we get is a metaphor, and the mismatch is part of the joke and part of the point. Beauty resists the kind of explanation a dictionary offers, so the poem answers in the only way that seems honest: by being brief, bright and over almost before we have taken it in. Herrick is known for short, finely turned poems, and this one is about as compact as a poem can be while still being a poem.
Its significance lies in how much it fits into twenty syllables. It turns an abstract noun into something we can see. It suggests that beauty is a moment and a relationship rather than a fixed possession. And it shows, for students especially, that a poem does not need length to raise a real question. Two lines, one image and one rhyme are enough to make us ask what we mean when we call something beautiful, and where exactly that quality is found.