Stereos (G4732) describes something solid and immovable โ the opposite of what wobbles or gives way. It describes solid food (as opposed to milk) for mature believers, a firm foundation, and the steadfast faith that cannot be shaken. The English word 'stereo' comes from this root (meaning solid, three-dimensional).
1 Peter 5:9 commands: 'Resist him [the devil], standing firm [stereos] in the faith.' 2 Timothy 2:19 declares: 'God's solid [stereos] foundation stands firm.' Hebrews 5:12-14 contrasts milk with solid [stereos] food โ the mature believer digests deep doctrine. The word pictures the kind of faith that doesn't shift when circumstances change because it is built on something immovable.