  Let $B$ be a half-integral symmetric matrix of size $n$ defined over
$\mathbb{Q}_p$. The Gross-Keating invariant of $B$ was defined by Gross and
Keating, and has important applications to arithmetic geometry. But the nature
of the Gross-Keating invariant was not understood very well for $n\geq 4$. In
this paper, we establish basic properties of the Gross-Keating invariant of a
half-integral symmetric matrix of general size over an arbitrary
non-archimedean local field of characteristic zero.
