Dot Percentage: Dot Percentage is expressed as a numerical value which equals the difference between the wanted value and the resulting value. For instance: if a page has a 50% flat tint as a background but after measuring the printed result, this flat tint is now 65%, the dot gain equals 15%.

Dot Gain: Dot gain is a phenomenon that causes printed material to look darker than intended. ... The optical and physical properties of the media and machines used both in preparing the job for print and the printing process itself cause this behavior.