notation - What does ! above = mean

Can someone please explain what the symbol $stackrel{!}{=}$, consisting of an exclamation mark (!) above an equals sign (=) means? Below is the example I'm trying to decipher:

---

The normalization factor is chosen such that in average, Dynamic Θ Time passes as fast as physical time. In practice it is determiend by the condition that the interval in Dynamic Θ Time corresponding to a 4-year reference period [$T_0$, $T_1$] should be of exactly the same length:

$T_1 - T_0 stackrel{!}{=} int_{T_0}^{T_1} a(t) dt$

• Next rt.representation theory - Non-smooth algebra with smooth representation variety
• Previous linear algebra - A generalization of Boolean matrix multiplication for order-3 tensors