What exactly is insertion gain?

[Sallie Frye]

Insertion gain is the difference between the individual's unaided 
response and his/her aided response.  It refers to measurements actually 
made in the ear with a probe microphone.  Insertion loss is the hearing 
loss that results from the insertion of a hearing aid or earmold into 
the ear.  The physical presence of the aid causes a loss that must be 
overcome by amplification.  At least these are how these terms are now 
used in the hearing aid industry.