EXCURSION FROM ASSIGNED INTERMEDIATE ALT BY FO OF A B737 DUE TO A MISUNDERSTANDING OF ATC CLRNC PASSED ON BY THE CAPT WHICH WAS THE CURRENT ALTIMETER SETTING AND NOT AN ALT CHANGE.

Narrative

APCHING BWI FROM THE E WE WERE CLRED TO 11000 FT; 250 KTS. WE CHKED ON WITH BWI APCH AT 11000 FT WITH THE ATIS INFO. WHEN APCH ACKNOWLEDGED OUR CHK-ON; THEY ISSUED A NEW ALTIMETER SETTING OF 30.00. WE ACKNOWLEDGED THE UNDATED INFO. AS I RESET THE 2 ALTIMETERS ON CAPT'S SIDE; I INADVERTENTLY SAID 3000 (3 THOUSAND) -- REFERRING TO THE ALTIMETER SETTING RATHER THAN A MORE APPROPRIATE VERBIAGE OF 30.00 (3 ZERO ZERO ZERO) ON THE ALTIMETER SETTING. NO OTHER CONVERSATION WAS ONGOING AT THE TIME AND WE WERE BOTH MONITORING APCH CTL. AT THE TIME I THOUGHT MY MEANING WAS CLR. SEVERAL MOMENTS LATER; I NOTICED WE WERE DSNDING OUT OF 11000 FT AND '3000' WAS SET IN THE ALT WINDOW. WE BEGAN A CLB BACK TO 11000 FT AS I RECONFIRMED OUR ASSIGNED ALT. OUR ASSIGNED ALT WAS CONFIRMED TO BE 11000 FT BUT A CLRNC TO 10000 FT WAS THEN ISSUED. AT THAT TIME WE WERE CLOSER TO 11000 FT THAN 10000 FT; SO WE LEVELED BACK AT 11000 FT AND RPTED LEVEL AT 11000 FT. APCH ACKNOWLEDGED. THERE WERE NO CONFLICTS WITH OTHER ACFT. AFTER LNDG; WE DISCUSSED THE INCIDENT FURTHER AND HOW A SIMILAR SIT MIGHT BE AVOIDED IN THE FUTURE. THE FO IS RELATIVELY NEW -- A MONTH OR TWO WITH THE ACR. I LEARNED THAT HE HAD MISINTERPED MY VERBALIZATION OF THE ALTIMETER SETTING AS A NEWLY ASSIGNED ALT AND THOUGHT THAT WHEN I RESTATED IT THAT I WANTED HIM TO RESET THE ALT SELECT WINDOW. HE HAD RESET THE ALT AND BEGAN A DSCNT. I EXPLAINED THAT EVEN THOUGH I THOUGHT MY MEANING HAD BEEN CLR THAT I WOULD BE MORE PRECISE IN MY VERBIAGE IN THE FUTURE. WE ALSO USED THIS EVENT AS A BASIS FOR DISCUSSION AS TO WHY THE PNF ALWAYS IS THE PERSON TO RESET THE ASSIGNED ALT AND THAT IT IS VERIFIED BY THE PF PRIOR TO ANY ALT CHANGES.