Is clopidogrel an anticoagulant

yes..
Heparin is indeed an anticoagulant. It disallows the formation of new clots and stops the progression of already formed clots (it can't decrease the size of these already formed clots though). So being a medication that alters the clotting ability of blood, it is considered an anticoagulant.

Also, calling it a, "blood thinner" is a misnomer since it does not actually "thin" the blood.
Heparin is indeed an anticoagulant. It disallows the formation of new clots and stops the progression of already formed clots (it can't decrease the size of these already formed clots though). So being a medication that alters the clotting ability of blood, it is considered an anticoagulant.

Also, calling it a, "blood thinner" is a misnomer since it does not actually "thin" the blood.