Hendeka (ἕνδεκα) is the Greek numeral eleven — eleven disciples, eleven sons. A compound of hen (one) + deka (ten). The word is theologically loaded because in the NT it almost always refers to the disciples after Judas's betrayal — the eleven who remained.
The "eleven" (hendeka) are a community defined by loss. After Judas, the twelve became eleven — the number marks an incompleteness, a wound. Yet it is to the hendeka that the risen Christ appears (Matthew 28:16), and it is the hendeka who receive the Great Commission. The incomplete number precedes Pentecost and the restoration: Acts 1 records the election of Matthias to restore the twelve. Incompleteness + commission = the pattern of the church — called to mission before fully restored, going forth with gaps still healing.
Hendeka appears only in the Gospels and Acts, always referring to the apostolic group. The number twelve had cosmic significance — twelve tribes, twelve months, twelve apostles corresponding to new-Israel. Eleven is Israel incomplete, the messianic community with a gap. The urgency to restore the twelve (Acts 1) before Pentecost is not bureaucratic but symbolic: the Spirit is poured out on a complete Israel.