Roz wrote:What exactly does "immaterial" mean?
Apart from two dubious Suttas in AN 3, that have no parallels, and that speak about the arūpadhathu - there seems to be only one occurrence of "arūpa" in the EBTs.
Speaking of the citta in Thag 19.1, this is what Mahāmoggallāna said:
You, incorporeal (citta,) far-traveller, lone-wanderer ...
Arūpa dūraṅgama ekacāri ...
speaks about losing the perception of forms (earth, water, etc.).
This is Jhana 5.
With the complete overstepping of perceptions of form (matter) - with the vanishing of perceptions (based) upon the organs of senses (viz. ajjhattikāni āyatanāni [including mano]) - not striving with the mind (manasa/mano) to perceptions of manifoldness (lit. (what is) differently than one), aware that ‘space is boundless,’ he attains and seizes distinctively, the field of experience of boundless space.
sabbaso rūpasaññānaṃ samatikkamā - paṭighasaññānaṃ atthaṅgamā nānattasaññānaṃ - amanasikārā ananto ākāsoti ākāsānañcāyatanaṃ upasampajja viharāti.
Note that saññā (perception) has the underlying meaning of "experience with acknowledgment" in pre-Buddhist Vedic texts.
Note also that the loss of "perception" of materiality (The four great elements (mahābhūtāna rūpa) and the forms out of (derived from) them (upādāya) - SN 12.2), is not the same as dwelling in "nothingness" in Jhāna 7:
By completely surmounting the base of infinite consciousness, aware that ‘there is nothing,’ a bhikkhu enters upon and dwells (distinctively,) in the base of nothingness.
sabbaso viññāṇañcāyatanaṃ samatikkamma ‘natthi kiñcī’ti ākiñcaññāyatanaṃ upasampajja viharāti.
Ākiñcaññā, which has the meaning of "nothing" here, means literally, that there is "not even from the cause" - "not even 'whence'" (na kiṃ cana).