What is the period of time that "Active" covers.

For instance on my team it shows active members as 1202, but if i sort the list of members by credit for last day, the number of members above 0 is only in th range of 750 to 800. If I look at average credit above 0 that shows 1501. So where does this active number come from ?

It depends whether you are looking BOINC Combined or only at 1 single project.

BOINC Combined: all crunchers with credit during last month are treated as active.
For projects: all crunchers with recent average credit higher than 0.1 are regarded as active.