Periplum is defined in the Oxford English Dictionary (OED) as having come from the poetry of Ezra Pound, specifically in The Pisan Cantos, Cantos LXXIV to LXXXIV of a larger work known collectively as The Cantos.
A periplum is a map or drawing that that shows how land looks from a point at sea. That is to say that a cartographer often draws maps from a bird-eye view and not from the perspective as the land would actually appear from the crow's nest or deck of a ship. Therefore a periplum would, theoretically, be drawn as if the cartographer were out to sea so that sailors could know which land or port they were approaching (Companion to The Cantos, Terrell; Chinese Characters as Traveling Metaphors in Canto LXXVII of Ezra Pound’s Pisan Cantos, and James Legge’s Book of Poetry 华英诗经 [Huá Yīng Shī Jīng], Tchou).
Pound uses the periplum as a figure to describe the form of the Cantos: not history from a historian's or philosopher's elevated point of view, but rather from the poet's point of view where the poet is a voyager navigating history personally.
As appears in the Pisan Cantos,
Periplum, not as land looks on a map
But as sea bord seen by men sailing.
(E. Pound Cantos LII-LXXI lix. 83)