Alkalinity roughly refers to the molar amount of bases in a solution that can be converted to uncharged species by a strong acid. For example, 1 mole of  in solution represents 1 molar equivalent, while 1 mole of  is 2 molar equivalents because twice as many H ions would be necessary to balance the charge. The total charge of a solution always equals zero. This leads to a parallel definition of alkalinity that is based upon the charge balance of ions in a solution. 
Certain ions, including Na, K, Ca, Mg, Cl, , and  are "conservative" such that they are unaffected by changes in temperature, pressure or pH.  Others such as  are affected by changes in pH, temperature, and pressure.  By isolating the conservative ions on one side of this charge balance equation, the nonconservative ions which accept or donate protons and thus define alkalinity are clustered on the other side of the equation.
This combined charge balance and proton balance is called total alkalinity. Total alkalinity is not (much) affected by temperature, pressure, or pH, and is thus itself a conservative measurement, which increases its usefulness in aquatic systems.  All anions except  and  have low concentrations in Earth's surface water (streams, rivers, and lakes).  Thus carbonate alkalinity, which is equal to  is also approximately equal to the total alkalinity in surface water.