OXFORD DICTIONARY
simultaneous, adj. (often foll. by with) occurring or operating at the same time.
simultaneous equations equations involving two or more unknowns that are to have the same values in each equation.
simultaneity n. simultaneously adv. simultaneousness n.
med.L simultaneus f. L simul at the same time, prob. after instantaneus etc.
THESAURUS
simultaneous
accessory, accompanying, agreeing, associated, attendant, attending, coetaneous, coeternal, coeval, coexistent, coexisting, coincident, coinciding, coinstantaneous, collateral, combined, concomitant, concurrent, concurring, conjoint, contemporaneous, contemporary, conterminous, correlative, coterminous, coupled, fellow, isochronal, isochronous, joined, joint, mutual, paired, parallel, synchronal, synchronic, synchronous, twin, unison, unisonous
ROGET THESAURUS
simultaneous
Synchronism
N
synchronism, coexistence, coincidence, simultaneousness, simultaneity, concurrence, concomitance, unity of time, interim, isochronism, contemporary, coetanian, synchronous, synchronal, synchronic, synchronical, synchronistical,
simultaneous, coexisting, coincident, concomitant, concurrent, coeval, coevous, contemporary, contemporaneous, coetaneous, coeternal, isochronous, at the same time, simultaneously, together, in concert, during the same time, in the same breath, pari passu, in the interim, as one, at the very moment, just as, as soon as, meanwhile.