# What is the universal definition of 'inertia' in fields?

In my physics class we recently did the Gravitational Field unit, and the idea of gravitational field strength was introduced: $$g=\frac{F}{m}=\frac{GMm}{r^2}\frac{1}{m}=\frac{GM}{r^2}$$ After doing that unit, now we're on to Electric Fields, and the idea of electric field strength was introduced: $$E=\frac{F}{q}=\frac{kQq}{r^2}\frac{1}{q}=\frac{kQ}{r^2}$$ I saw the main similarity; mass was replaced with charge (and a different proportionality constant). My teacher explained that phenomenon in general for all fields as: $$Field \,\,Strength=\frac{F}{Intertia}$$ He said in the gravitational field, the inertia is mass. In electric fields, the inertia is charge. My understanding of inertia was the classical mechanics one; the property of matter that resists change in motion. But my teacher explained that each field has it's own 'inertia'. I've been searching the web about specifically this, the formal definition of inertia across all fields, and have not come across anything. Does such a definition even exist?