  Let k be a separably closed field. Let K_i=[A_i \to B_i] (for i=1,2,3) be
three 1-motives defined over k. We define the geometrical notions of extension
of K_1 by K_3 and of biextension of (K_1,K_2) by K_3. We then compute the
homological interpretation of these new geometrical notions: namely, the group
Biext^0(K_1,K_2;K_3) of automorphisms of any biextension of (K_1,K_2) by K_3 is
canonically isomorphic to the cohomology group Ext^0(K_1 \otimes K_2,K_3), and
the group Biext^1(K_1,K_2;K_3) of isomorphism classes of biextensions of
(K_1,K_2) by K_3 is canonically isomorphic to the cohomology group Ext^1(K_1
\otimes K_2,K_3).
