The Greek adjective elachistos is the superlative form of mikros (small), meaning the smallest, least, or most insignificant. In the New Testament it is used both literally (smallest matters) and theologically (least in the kingdom).
Jesus uses elachistos to teach radical reversals of value: 'Whoever does and teaches these commands will be called great in the kingdom of heaven' (Matthew 5:19). In Matthew 25:40, service to 'the least of these' is service to Christ himself — making the elachistos the site of encounter with Jesus.
Theologically, elachistos captures the kingdom inversion: what the world considers least is what God honors most. Paul calls himself 'less than the least of all the Lord's people' (Ephesians 3:8) — modeling the humility that opens the door to grace.