"firmness ..."See in text(A Valediction: Forbidding Mourning)
“Firmness” here refers to steadfastness or faithfulness. “Just,” meaning both righteousness and accuracy, becomes dependent on this firmness. In other words, the speaker suggests to his mistress that if she is able to remain firm, faithful, and steadfast, then his circle, his journey back to her, will be righteous and accurate.
"fix'd foot..."See in text(A Valediction: Forbidding Mourning)
If you were drawing a circle with a mathematical compass, you would “fix” one foot to one point on a piece of paper and then rotate the other foot around it in order to draw the circle. The “free” foot that is moving, always circles around the fixed foot because they are attached.
"Inter-assurèd ..."See in text(A Valediction: Forbidding Mourning)
This is a compound word that Donne seems to have made up as it historically only appears in this poem. “Inter” means between; “assured” means to secure, promise, or make safe. Thus, this word suggests that there is a promise or security between the speaker and his mistress that is made up of their minds rather than their bodies. This once again situates their love outside the human realm of the body.
"Those things which elemented ..."See in text(A Valediction: Forbidding Mourning)
“Elemented” in this context means created or produced. “Those things,” which seem to create the love, are vague here. However, since the speaker has characterized the lovers as of the earth, “those things” can be inferred as referencing the body or carnal acts. In other words, their love is entirely made up of their bodies.
"admit
Absence..."See in text(A Valediction: Forbidding Mourning)
The “sublunary lovers” cannot “admit” (permit or accept) “absence” (distance from one another) because their love depends on their ability to touch. The speaker uses this description to qualify his characterization of the lovers’ love as dull, since their love cannot survive if their bodies are not close to each other. By outlining this impure love, the speaker implicitly makes his own love seem more pure.
"sublunary..."See in text(A Valediction: Forbidding Mourning)
“Sublunary” means that which lies between the earth and the moon’s orbit, as in on the surface of the earth. The speaker uses this cosmic picture to both transition from his conceit about the spheres and refer to ordinary lovers who are subject to change because their love is directly related to the inconstant moon.
"laity..."See in text(A Valediction: Forbidding Mourning)
By this the speaker means someone who is not of the clergy, or religious orders. The speaker uses this word to distinguish his love from the lay or mortal world and implicitly make the lovers part of a religious or other worldly realm.
"Twere..."See in text(A Valediction: Forbidding Mourning)
This is a contraction of “it were.” Donne’s need to shorten these two already short words comes from the meter, or rhythm, of the line which is written in iambic tetrameter, meaning there are four stressed syllables per line. “‘Twere” fits better because the stresses in this poem fall on the second syllable of the line. Since the “pro” of profanation must be stressed, “it were” would not work metrically because then both “were” and “pro” would be stressed throwing off the meter of the line.
"melt..."See in text(A Valediction: Forbidding Mourning)
By melt Donne means to disintegrate or liquefy. This metaphor is another reference to Carpe Diem poetry as the speaker here suggests that the two lovers become one. However, “melt” at this time also meant to disperse into particles, to vanish or disappear. Thus, the line proposes that the lovers simultaneously become one and move away from each other. This dual meaning foreshadows the conclusion Donne will draw at the end of the poem.