A B737 FO WAS TOLD TO MAINTAIN 250 KTS BY C90. THE FO DID NOT UNDERSTAND THIS INSTRUCTION AND VIOLATED HIS AIRSPD RESTR.

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AT 1000 FT AFL ON TKOF; I COMMANDED VNAV. TWR; AT ABOUT 3500 FT; TOLD US 'MAINTAIN 250 KTS; CONTACT DEP.' SINCE A BIG DEP PUSH WAS IN PROGRESS; IN MY MIND; I TOOK THE 250 (MISTAKENLY) AS 250 KTS ASAP. WE CONTACTED DEP WITH CURRENT ALT AND ASSIGNED ALT ONLY. DEP ASSIGNED US 13000 FT. ATC SUBSEQUENTLY INSTRUCTED THE ACFT IN FRONT OF US 'BEST FORWARD SPD ASAP OUT OF 10000 FT.' AT 10000 FT; I ACCELERATED TOWARD THE VNAV COMPUTED SPD OF ABOUT 334 KTS. AROUND 320 KTS AND 11300 FT; ATC SAID 'MAINTAIN 250 KTS UNTIL ADVISED.' I PROMPTLY BROUGHT THE PWR BACK AND PITCHED UP TO SLOW QUICKLY; REMEMBERING THE 'MAINTAIN 250 KTS' PREVIOUSLY ISSUED. IT WOULD HAVE HELPED IF WE WERE ORIGINALLY INSTRUCTED TO 'MAINTAIN 250 KTS UNTIL ADVISED' IT PAINTS A MORE CONCRETE PICTURE IN MY MIND. I'M NOT DUCKING MY RESPONSIBILITY ON THIS; BUT 'UNTIL ADVISED' WOULD MORE QUICKLY PROMPT ME TO CHANGE/COMMAND THE FLT GUIDANCE TO NOT ACCELERATE AT 10000 FT (IE; VNAV SPD INTERVENTION 250 KTS OR LEVEL CHANGE WITH SPD SET AT 250 KTS). ALSO; WHEN WE CHKED ON TO DEP; WE DIDN'T STATE THAT WE WERE ASSIGNED/RESTR TO 250 KTS. SO; A CONFIRMATION FROM ATC MAY HAVE HELPED.