What is Annuity measure?

For a swap with fixed payments at $T_1 < \dots < T_n$ and accrual fractions $\delta_i$, the annuity measure $\mathbb Q^A$ is the equivalent martingale measure for the numeraire $A_t = \sum_i\delta_iP(t, T_i)$, the annuity of One Quant Book 2, chapter 9.