Debye-Huckel limiting law I encountered this equation when studying the Electrolyte solutions chapter in my analytical Chemistry textbook, Actually the terms in summation are giving a physical clue of what $I$ is, But what's the point of that $\frac{1}{2}$ ?
I've searched the Internet and a molecular Thermodynamics book that I have but I found nothing but the equation itself.

$I=\dfrac{1}{2}\sum_{i}m_{i}z_{i}^{2}$
$I$ is the ionic strength of a solution, and is a measurement of the concentration of ions in a solution. $m_i$ is the concentration of the ion i, and $z_i$ is the charge of that ion. The $\frac{1}{2}$ in front of the summation is a result of the fact that both positive and negative ions are considered when calculating the ionic strength. $m_i$ could either be a measurement of Molarity or molality, and your book should state which definition you are using (I assume molality due to the lowercase m).