The Collaborative International Dictionary of English v.0.48:

Synchronous \Syn"chro*nous\, a. [Gr. ?; sy`n with + ? time. Cf.
   Chronicle.]
   Happening at the same time; simultaneous. --
   Syn"chro*nous*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

synchronous
    adj 1: occurring or existing at the same time or having the same
           period or phase; "recovery was synchronous with therapy"-
           Jour.A.M.A.; "a synchronous set of clocks"; "the
           synchronous action of a bird's wings in flight";
           "synchronous oscillations" [syn: synchronous,
           synchronal, synchronic] [ant: asynchronous]
    2: (digital communication) pertaining to a transmission
       technique that requires a common clock signal (a timing
       reference) between the communicating devices in order to
       coordinate their transmissions [ant: asynchronous]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "synchronous":
   accompanying, accordant, according, affirmative, agreeable,
   agreeing, answerable, associate, associated, assonant, assonantal,
   at one, at one with, attuned, blended, blending, chiming, coacting,
   coactive, coadunate, coetaneous, coeval, coexistent, coexisting,
   coherent, coincident, coinciding, collaborative, collective,
   collusive, combined, combining, commensurate, compatible,
   concerted, concomitant, concordant, concurrent, concurring,
   conformable, congenial, congruent, congruous, conjoint,
   consentaneous, consentient, consilient, consistent, consonant,
   conspiratorial, contemporaneous, cooperant, cooperating,
   cooperative, coordinate, correspondent, corresponding, coworking,
   en rapport, equivalent, harmonic, harmonious, harmonizing,
   homophonic, in accord, in agreement, in chorus, in concert,
   in concord, in phase, in rapport, in step, in sync,
   in synchronization, in tempo, in time, in tune, in unison,
   inaccordance, inharmony, joint, like-minded, meeting, monodic,
   monophonic, of a piece, of like mind, of one mind, on all fours,
   on the beat, parasitic, positive, proportionate, reconcilable,
   saprophytic, self-consistent, simultaneous, symbiotic, symphonious,
   synchronal, synchronic, synchronized, synergetic, synergic,
   synergistic, tuned, unanimous, uniform, unisonant, unisonous,
   united, uniting



The Free On-line Dictionary of Computing (30 December 2018):

synchronous

    1. Two or more processes
   that depend upon the occurrences of specific events such as
   common timing signals.

   2. Occurring at the same time or at the same rate or with a
   regular or predictable time relationship or sequence.

   Opposite: asynchronous.

   (1996-04-11)