We initiate the theory of a quadratic form
q over a semiring
R, with a view to study tropical linear algebra. As customary, one can write
where
b is a
companion bilinear form. In contrast to the classical theory of quadratic forms over a field, the companion bilinear form need not be uniquely defined. Nevertheless,
q can always be written as a sum of quadratic forms
q=qQL+蟻, where
qQL is
quasilinear in the sense that
qQL(x+y)=qQL(x)+qQL(y), and
蟻 is
rigid in the sense that it has a unique companion. In case that
R is supertropical, we obtain an explicit classification of these decompositions
q=qQL+蟻 and of all companions
b of
q, and see how this relates to the tropicalization procedure.