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| a printed punctuation mark (‽), available only in some typefaces, designed to combine the question mark (?) and the exclamation point (!), indicating a mixture of query and interjection, as after a rhetorical question. |
| the offspring of a zebra and a donkey. |
| sound barrier | |
| —n | |
| (not in technical usage) sonic barrier, Also called: transonic barrier a hypothetical barrier to flight at or above the speed of sound, when a sudden large increase in drag occurs | |
| sound barrier
The sharp increase in drag experienced by aircraft approaching the speed of sound. Also called sonic barrier. |
The sudden increase in air resistance that occurs when an aircraft approaches the speed of sound. This is also called the sonic barrier.