The Hebrew noun nichoach literally means a soothing or pleasing aroma — specifically the scent of a sacrificial offering that was 'pleasing to the LORD.' It appears almost exclusively in the phrase 'a soothing aroma to the LORD' (reyach nichoach la-YHWH).
Nichoach is the sensory language of sacrifice. From Noah's burnt offering (Genesis 8:21) through every Levitical sacrifice, the phrase 'a pleasing aroma to the LORD' (reyach nichoach) communicated that God received and accepted the offering. But the prophets were clear that God's true nichoach was not the smell of burning animals but the sweet fragrance of obedience, justice, and love (Amos 5:21-24). Paul brings the metaphor to its fullness: Christ 'gave himself up for us as a fragrant offering and sacrifice to God' (Ephesians 5:2) — the ultimate nichoach, accepted once for all. Believers carry this aroma as 'the pleasing aroma of Christ' in the world (2 Corinthians 2:15).