理想溶液是溶质与溶剂混合为溶液时，既不放热，也不吸热，溶液体积适等于溶质体积和溶剂体积之和。

实际存在的溶液均不能合于理想溶液的定义，但是溶质分子结构和性质越是与溶剂分子相接近，混合后的溶液行为就越和理想溶液相近。一般来说，稀溶液的行为较浓溶液的行为更加接近理想溶液的行为。

在化学中代表该溶液溶解时的溶解热（焓的改变量）为零；溶解热越接近零，则该溶液的行为就越为「理想」。这个概念主要是奠基于化学热力学及其应用。另一种说法是溶质与溶质之间、溶剂与溶质之间、溶剂与溶剂之间的作用力都相等，则为理想溶液。

In chemistry, an ideal solution or ideal mixture is a solution with thermodynamic properties analogous to those of a mixture of ideal gases. The enthalpy of mixing is zero as is the volume change on mixing; the closer to zero the enthalpy of solution is, the more "ideal" the behavior of the solution becomes. The vapor pressure of the solution obeys Raoult's law, and the activity coefficient of each component (which measures deviation from ideality) is equal to one.