Tritos (G5154) means third — the ordinal numeral. In its NT context it is overwhelmingly significant as 'the third day' (tē tritē hēmera), the day of Christ's resurrection — the phrase Paul cites as foundational kerygma in 1 Corinthians 15:4.
The resurrection 'on the third day' (tē tritē hēmera) appears 12+ times in the NT and is rooted in OT typology: Jonah in the fish three days (Matthew 12:40), Isaac received back on the third day (Genesis 22:4, Hebrews 11:19), Hosea's 'on the third day he will restore us' (Hosea 6:2). The third day is God's day of reversal, resurrection, and new beginning. Every Sunday — the first day, the eighth day, the day after the Sabbath — is a celebration of the tritos.