On arrival at a major UK airport, we entered a hold at FL150 with 30 minutes delay due to strong winds. We stepped down in the hold pattern by approximately 1000 feet each hold, i.e. 150, 140, 130, 120, 110, 100, 90. We were transferred to the Director at around FL100. The next clearance was understood as DESCEND FL80, which was our next lower level. At or near FL80, ATC asked if we had TURNED to heading 080. Need I describe the dreadful feeling? Mortified! I apologized on the R/T. ATC responded, No problem, gave updated heading and clearance for further descent. However, that airport is not the place to be at the wrong level and heading on a busy and rough Sunday night! Having given the incident much thought in the days following the incident, I believe that a major contributing factor was the expectation, quite reasonably in a sense, of further descent to FL80 and hearing what we thought we should hear, thus confusing heading with cleared level. As vulnerable as one can be on a new type, it could have happened on my previous type (23 years, 13,000 hours). In addition, I had a good first officer. 1. What does stepped down in the first paragraph mean?