Union sits next to 4 phonetic neighbors — Orion, Onyx, and Owen are the closest matches by sound. Most share Union's Latin unio (oneness)-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"the act of joining together, unity, oneness"
"from Jerez (the wine region)"
"night, dark beauty (beautiful night)"
"from the round hill"
"God will add, God will increase"
"plural of ovum (biological)"
"serpent-like, winding"
Similar sound: These names share phonetic patterns with Union — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Latin unio (oneness) origins, giving them the same cultural heritage as Union.
Rhythm: Union is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.