In thermodynamics, activity refers to an ‘effective’ concentration, which takes nonideal behavior into account. Thus, ion-to-ion interactions, such as charge shielding, may affect the activity by attractive or repulsive intermolecular forces. The activity of a dissolved species is calculated as a_i = γ_im_i where γ_i is the activity coefficient and m_i is the molality. Activity is the term used to describe concentrations in solutions, whereas fugacity describes the activity of a gas.