# Terminology of rho electroproduction

Dear Forum,

e + p -> e + p + rho

One of the commonly used kinematic variable is tprime, defined as t-tmin.

The definition I could find for tmin is that it is the minimal value of t kinematically
necessary to put rho and proton system on shell.

However I still do not know how it is calculated and what does 'putting the system
on shell' mean. More explanation will be greatly appreciated. Thanks for any response
you might have.