Lewis & Short

2. ūnĭo, ōnis, f. and m. [unus].

  1. I. Fem
    1. A. The number one, oneness, unity (eccl Lat.): decas decimā unione completur, Hier. in Amos, 2, 5, 5; Tert. Monog. 4; id Res. Carn. 2 fin.
    2. B. A unity, union (late Lat.): Maria Dei unione fecunda, Hier. Ep 22, 19; 18, 14.
  2. II. Transf., concr.
    1. A. Masc., a single large pearl (cf. margarita), Plin. 9, 35, 56, § 112; 9, 35, 59, § 122; Sen. Ben. 7, 9, 4; Mart. 8, 81, 4; 12, 49, 13.
      Fem.: Cleopatranae, Treb. XXX. Tyrann. 32 fin.
    2. B. Fem., a kind of single onion caepam, quam vocant unionem rustici. Col. 12, 10, 1.