A Poem’s form is that which can be wholly described by a fully abstract map. For instance, by the number of words and lines in it, and the length of the latter. It can be very detailed. It ought to be as objective as possible, but when most complete will require a consensus of experts[x] to validate some of its details. Take, for instance, what I deem “the Classical Haiku in English” and take its form to be three lines, the first and third of which contain five syllables and the second seven syllables; and whose words (here subjectivity leaks in) denote two or more images at least one of which is from Nature, and which are in tension with one another, a tension whose resolution results in a “haiku moment.” It would take twenty pages or more to say (approximately) what a haiku moment is. For now suffice it to say it is a feeling of sudden, consequential (celebratory, I contend) illumination about existence.
The definition of a poem’s form will never satisfy everyone, but I believe that a great majority of poetry scholars and laymen will agree on enough of it to allow reasonably profitable discussion. For instance, perhaps no one will agree entirely on my definition of the form of a classical haiku (in English) form, but I believe most haiku-lovers will find it close enough for discussion.
Every poem has a form, but not every poem is what I call “classiformular,” by which I mean having a form shared by numerous other poems such as the sonnet and haiku. I suspect it’s impossible for a poem not to share some abstract quality with any other poem, but certainly many free verse poems have less in common than they have in common with other poems, of I call them “idioformular.”
A poem’s form contains a poem’s contents—including itself—including, that is, what it connotes by its allusion to all other poems sharing its form—i.e., Basho is in every classical haiku in any language. Even the most idioformular poem’s form will connote freedom or wildness, and thus become a portion of the poem’s content.
[x] My definition of an expert in a given subject: “One who has produced a full-length coherent book or the equivalent on the subject that follows most of the established methodology of scholars seriously involved with the subject (e.g., logic) and definitions (although redefinition, if stated, should be permissible).” This definition may need work, but it should do for the purposes of this entry.