Alalētos (G215) means that which cannot be spoken or uttered — from alpha-privative + laleō (to speak). It appears only in Romans 8:26, where the Spirit intercedes for us with 'groanings too deep for words' (stenagmois alalētois). The inexpressible is the Spirit's own prayer on behalf of the helpless believer.
Romans 8:26 is one of the most comforting verses about prayer. When believers do not know how to pray or what to say, the Spirit himself takes up the burden with groanings that transcend language. The word alalētos — inexpressible — captures this: some prayer is too deep, too desperate, too holy for human words. The Spirit translates the inarticulate groaning of the heart into perfect intercession before the Father.