Mas-Colell, Whinston and Green, in Microeconomic Theory (third edition), postulate the concept of an index for an excess demand vector, which is later used in the Index Theorem:
A regular equilibrium of the economy is defined as the following:
$D$ denotes derivative (here, in respect to price), while $z$ denotes the excess demand matrix:
Where $p$ denotes price and $\omega$ denotes endowment.
What does the concept of index means?