From the longer Wikipedia page [1].

Hypomnema (Greek. ὑπόμνημα, plural ὑπομνήματα, hypomnemata), also spelled hupomnema, is a Greek word with several translations into English including a reminder, a note, a public record, a commentary, a draft, a copy, and other variations on those terms.

The hypomnemata constituted a material memory of things read, heard, or thought, thus offering these as an accumulated treasure for rereading and later meditation. They also formed a raw material for the writing of more systematic treatises in which were given arguments and means by which to struggle against some defect (such as anger, envy, gossip, flattery) or to overcome some difficult circumstance (a mourning, an exile, downfall, disgrace).

Michel Foucault uses the word in the sense of "note", but his translators use the word "notebook", which is anachronistic (see codex and wax tablet). Concerning Seneca's discipline of self-knowledge, Foucault writes: "In this period there was a culture of what could be called personal writing: taking notes on the reading, conversations, and reflections that one hears or engages in oneself; keeping kinds of notebooks on important subjects (what the Greeks call 'hupomnemata'), which must be reread from time to time so as to reactualize their contents."

The Oxford Dictionary of Byzantium, ed. Alexander P Kazhdan, reference is [2].