What does MN 18's "delineate a delineation" phrase mean?
Ven. Bodhi's MN 18 version:


  "When there is the eye, a form, and eye-consciousness, it is possible to point out the manifestation of contact.233 When there is the manifestation of contact, it is possible to point out the manifestation of feeling. When there is the manifestation of feeling, it is possible to point out the manifestation of perception..."


and footnote:


  The Pali idiom phassapaññattiª paññ›pessati, in which the verb takes an object derived from itself, is difficult. Ñm originally rendered “that one will describe a description of contact.” “To point out a manifestation” is less literal, but it should do justice to the meaning without jeopardising intelligibility. MA says that this passage is intended to show the entire round of existence (va˛˛a) by way of the twelve sense bases; §18 shows the cessation of the round (viva˛˛a) by the negation of the twelve sense bases.


Prof. Piya Tan shares similar explanation:


  So vat‟āvuso cakkhusmi sati rpe sati cakkhu,viāe sati phassa,paatti paāpessatîti hāna. Comy says that this passage shows the entire round of existence (vaā) by way of the 12 sense-bases. The next section [§18] shows the cessation of the round (vivaa) by the negation of the 12 sense-bases. (MA 2:78). The structure paatti paāpessati (lit “he describes the description,” “he defines the definition”) is idiomatic, meaning simply “he describes; he defines.” Paraphrased, this sentence may also read “It is possible to define contact as the meeting of sense-organ, sense-object and sense-consciousness.”