teaching: both the act of instructing and the body of instruction that results
The word names teaching in two connected ways: the living act of instructing someone, and the settled body of what is taught. English usually keeps these apart, calling the first "teaching" or "instruction" and the second "doctrine," but one Greek word covers the whole arc, from the teacher's work to the content that lasts.
The word itself is neutral. Scripture uses it for God's own teaching and for "sound teaching," but also, with the very same word, for "the teachings of men" (Matthew 15:9; Mark 7:7) and even "teachings of demons" (1 Timothy 4:1). Whether the teaching is true or false is carried by the words around it, not by this word.
Most occurrences fall in the letters to Timothy and Titus, where "sound teaching" (healthy, wholesome teaching) marks reliable Christian instruction off from novelty and error. There the line between the act and the content blurs on purpose: a faithful teacher and a faithful message stand or fall together.
It is the noun paired with the verb διδάσκω "to teach" and the διδάσκαλος who does the teaching. Its near-twin διδαχή overlaps heavily, but these letters prefer this word for the trustworthy body of instruction the church is to guard.