infrequent

Infrequent, a.

Etymology: [L. infrquens : cf.F. infrequent. See In- not, and Frequent.]

Seldom happening or occurring; rare; uncommon; unusual. The act whereof is at this day infrequent or out of use among all sorts of men. Sir T. Elyot.

Shades of meaning

Fernald has no entry of its own for infrequent; he lists it under rare.

Where he writes about it:

Unique is alone of its kind; rare is infrequent of its kind; great poems are rare; "Paradise Lost" is unique. — from rare