An expression is called "well-defined" (or "unambiguous") if its definition assigns it a unique interpretation or value. Otherwise, the expression is said to not be well-defined or to be ambiguous.
For example, the expression 
(the product) is well-defined
if 
, 
, and 
are integers. Because integers are associative,
has the same value whether it is interpreted
to mean 
or 
. However, if 
, 
,
and 
are Cayley numbers, then the expression 
is not well-defined, since Cayley
numbers are not, in general, associative, so
that the two interpretations 
and 
can be different.
Sometimes, ambiguities are implicitly resolved by notational convention. For example, the conventional interpretation of 
is 
, never 
, so that the expression 
is well-defined even though exponentiation is nonassociative.
The term "well-defined" also has a technical meaning in field of partial differential equations. A solution to a partial differential equation that is a continuous function of its values on the boundary is said to be well-defined. Otherwise, a solution is called ill-defined.
