A is B to within a constant factor
A is between (B times a constant) and (B times a constant) where the first constant and the second constant are not necessarily the same.
From page 45 of the third edition (Cormen, Thomas H., et al. Introduction to Algorithms, MIT Press, 2014--via ProQuest Ebook Central):
Figure 3.1(a) gives an intuitive picture of functions f(n) and g(n), where
f(n) = Θ(g(n)). For all values of n at and to the right of n0, the value of f(n)
lies at or above c1g(n) and at or below c2g(n). In other words, for all n ≥ n0, the function f(n) is
equal to g(n) to within a constant factor.
The key is in
the value of f(n)
lies at or above c1g(n) and at or below c2g(n)
where c1 and c2 are defined by the caption of Figure 3.1 as being positive constants.
So we can rephrase as
the value of f(n) is greater than or equal to a constant value times g(n) and less than or equal to a constant value times g(n).