decade
n.
5LME < OFr < LL decas (gen. decadis) < Gr dekas < deka, TEN6
1 a group of ten
2 a period of ten years; esp., in the Gregorian calendar
a) officially, a ten-year period beginning with the year 1, as 1921-1930, 1931-1940, etc. b) in common usage, a ten-year period beginning with a year 0, as 1920-1929, 1930-1939, etc.
3 a division of the rosary consisting of one large bead and ten small beads

decennial
adj.
5< L decennium (see fol.) + 3AL6
1 happening every ten years
2 lasting ten years >
n.
a tenth anniversary or its commemoration
de[cen4ni[al[ly
adv.

decennium
n.,
pl. 3ni[ums or 3ni[a 73!8 5L < decennis: see DECENNARY6 DECADE (sense 2)
Dear WW: there are the three definitions. I'm not sure what "the second sense" referred to.
Forgive me for having delayed reply.