HAWKER 400XP FLT CREW HAS FUMES IN COCKPIT; DECLARES AN EMER AND DIVERTS TO ZZZ.

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PREFLT; TAXI; TKOF; AND INITIAL DEP FROM ZZZ1 WERE UNEVENTFUL. HOWEVER; DURING CLBOUT; AN UNUSUAL ODOR WAS DETECTED IN THE COCKPIT. AS CLBOUT WAS CONTINUED; THE ODOR INTENSIFIED; AND I DECIDED TO RETURN AND CHANGED OUR DEST FROM ZZZ2 TO ZZZ3. AN EMER WAS NOT DECLARED AT THIS TIME. AT NO TIME WAS ANY VISIBLE SMOKE DETECTED IN THE COCKPIT BY EITHER CREW MEMBER AND THERE WAS ALSO NO INDICATION OF ANY ONBOARD SYS MALFUNCTION IN THE COCKPIT -- OTHER THAN THE ODOR. DURING DSCNT WHILE ENRTE TO THE ZZZ4 VOR; AT AROUND 10000 FT; THE ODOR BEGAN TO INTENSIFY AND PERMEATE THE COCKPIT. THE ENVIRONMENTAL CTL SYS WAS TURNED OFF; BUT THE ODOR CONTINUED TO BE PRESENT. BECAUSE THE ODOR WAS INTENSIFYING AND THE SOURCE WAS UNKNOWN; AS A PRECAUTION I DECIDED TO DECLARE AN EMER. AT THAT POINT; ZZZ3 WAS; IN MY MIND; THE CLOSEST SUITABLE ARPT FOR THE SITUATION AS IT EXISTED. WHILE THE PROB WAS REFERRED TO AS 'SMOKE' WITH ATC; THERE WAS NEVER ANY VISIBLE SMOKE IN THE COCKPIT. THE TERM WAS PROBABLY A POOR CHOICE OF WORDS ON MY PART; AT A TIME OF A HIGH WORKLOAD; BUT IN THE INTEREST OF BREVITY WAS THE TERM I USED. A MORE CORRECT VERBIAGE WOULD HAVE BEEN 'AN UNUSUAL ODOR FROM AN UNKNOWN SOURCE' IN THE COCKPIT. THE ODOR BEGAN TO SUBSIDE SOMEWHAT JUST PRIOR TO LNDG; AND THIS INFO WAS RELAYED TO ATC. AFTER LNDG; I FELT IT WAS PRUDENT TO TERMINATE THE EMER; AND TAXI TO THE FBO -- A VERY SHORT TAXI. MY VISUAL INSPECTION OF THE ACFT AFTER LNDG REVEALED NO APPARENT SOURCE FOR THE ODOR; NOR DID ANY APPARENT VISUAL ABNORMALITIES EXIST EITHER INSIDE OR OUTSIDE THE ACFT. THE ACFT WENT INTO MAINT FOR TROUBLESHOOTING. NO DEV FROM REGULATORY DIRECTIVES OCCURRED DURING THIS EVENT; AND ALL ATC CLRNCS WERE ADHERED TO WHILE CONDUCTING THE ARR AND LNDG.