In C terminology, "object" refers to any kind of data, whether it's a literal or a value stored in a variable. For example, you might here about "object-like macros" which just mean preprocessor macros that don't take any arguments and can be used in place of variables. For example
#define STRING_LENGTH 256
is defining an "object-like" macro. In contrast,
#define CUBE(n) ((n)*(n)*(n))
is defining a "function-like" macro.