once/at once
ephapax marks either a single, decisive occurrence or people gathered at one time in the attested verses.
The adverb most often marks an action or event presented as occurring in a single, non-repeated instance: offerings, entry, and death are described this way in Hebrews and Romans. In 1 Corinthians 15:6, it instead describes more than five hundred brothers as present together at one time.
Senses in use
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A single, non-repeated act or event · dominant
In Hebrews 10:10 the APB renders sanctification through the offering of Jesus Christ's body as occurring "once for all," while Hebrews 7:27 places the term with his offering of himself rather than the daily sacrifices. Hebrews 9:12 similarly renders his entry into the holy place "once for all," and Romans 6:10 applies it to the death he died to sin.
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Together at one time · rare
In 1 Corinthians 15:6, the adverb describes the appearance to more than five hundred brothers in the same occasion; the APB renders the clause "more than five hundred brothers at once."
In the Anselm Project Bible: The APB renders the term as "once for all" in Hebrews and Romans, but as "at once" in 1 Corinthians 15:6. These renderings confirm the clauses' presentation rather than supply a separate definition by themselves.