  We show, under general assumptions which are well satisfied in relativistic
heavy-ion collisions, that the geometric relation of centrality c to the impact
parameter b, namely c ~ pi b^2/sigma_inel, holds to a very high accuracy for
all but most peripheral collisions. More precisely, if c(N) is the centrality
of events with the multiplicity higer than N, then b is the value of the impact
parameter for which the average multiplicity <n(b)> is equal to N. The
corrections to this geometric formula are of the order (Delta n(b)/<n(b)>)^2,
where Delta n(b) is the width of the multiplicity distribution at a given value
of b, hence are very small. In other words, the centrality effectively measures
the impact parameter.
