1.  thunderclap(n = noun.event) - a single sharp crash of thunder;
is a kind of thunder

2.  thunderclap(n = noun.event) bombshell, thunderbolt - a shocking surprise; "news of the attack came like a bombshell"
is a kind of surprise

   A sharp burst of thunder; a sudden report of a discharge of atmospheric electricity. Spenser. [1913 Webster]
"When suddenly the thunderclap was heard." [1913 Webster]

thunderclap, n.
1 a crash of thunder.
2 something startling or unexpected.

N  inexpectation, non-expectation, false expectation, miscalculation, surprise, sudden burst, thunderclap, blow, shock, start, bolt out of the blue, wonder, eye opener, unpleasant surprise, pleasant surprise, nonexpectant, surprised, unwarned, unaware, off one's guard, inattentive, unexpected, unanticipated, unpredicted, unlooked for, unforeseen, unhoped for, dropped from the clouds, beyond expectation, contrary to expectation, against expectation, against all expectation, out of one's reckoning, unheard of, startling, surprising, sudden, unpredictable, unforeseeable (unknowable), abruptly, unexpectedly, surprisingly, plump, pop, a l'improviste, unawares, without notice, without warning, without a 'by your leave', like a thief in the night, like a thunderbolt, in an unguarded moment, suddenly, Int, heydey!, little did one think, little did one expect, nobody would ever suppose, nobody would ever think, nobody would ever expect, who would have thought? it beats the Dutch.