Meaning of ACTIVITY in English

"The activity of a dissolved species in solution is the ""effective"" concentration of that species. In an ""ideal"" solution, the molecules in the solution do not interact with each other and the concentration and the activity are identical. This is the case for very dilute solutions. In a ""real"" solution, there is a certain interaction between the molecules resulting in a diminished ""activity"" of the molecules toward the outside world, and the solution behaves like it would contain lower concentration of the dissolved species than it actually does. The activity can be expressed as the product of an ""activity coefficient"" and the concentration. "

Electrochemistry English dictionary.      Английский словарь электрохимии.