From Medieval Latin simultaneus, from simultim, from Latin simul; compare similar.

• (British) IPA: /ˌsɪm.əlˈteɪ̯.ni.əs/, /ˌsɪm.əlˈteɪ̯.njəs/
• (America) IPA: /ˌsaɪ̯.məlˈteɪ̯.ni.əs/, /ˌsaɪ̯.məlˈteɪ̯.njəs/

simultaneous (not comparable)

1. Happening at the same moment.
   • 1992, Rudolf M[athias] Schuster, The Hepaticae and Anthocerotae of North America: East of the Hundredth Meridian, volume V, Chicago, Ill.: Field Museum of Natural History, →ISBN, pages 3-4:

     As with the Lejeuneaceae, this pattern of massive speciation appears to be correlated with the Cretaceous explosion of the angiosperms and the simultaneous creation of a host of new microenvironments, differing in humidity, light intensity, texture, etc.

2. (mathematics, of a set of equations) To be solved for the same values of variables.

• (antonym(s) of “happening at the same moment”): sequential, asynchronous, metachronous

• French: simultané
• German: gleichzeitig, simultan
• Italian: simultaneo, concomitante
• Portuguese: simultâneo
• Russian: одновре́ме́нный
• Spanish: simultáneo