From The Collaborative International Dictionary of English v.0.48 [gcide]:
Hebdomad \Heb"do*mad\, n. [L. hebdomas, -adis, Gr. "ebdoma`s the
number seven days, fr. ? seventh, ? seven. See Seven.]
A week; a period of seven days. [R.] --Sir T. Browne.
Hebdomadal
From WordNet (r) 3.0 (2006) [wn]:
hebdomad
n 1: any period of seven consecutive days; "it rained for a
week" [syn: week, hebdomad]