Wikipedia
MID-LIFT (also mid-lift) is a design and installation of a Rocker arm on the internal combustion engine, of an Over-Head-Valve (OHV) design. The "MID-LIFT" principle became known as a term within a US Patent, #4,365,785, issued to Jim Miller in 1982, also referred to as the MID-LIFT Patent. For many years MID-LIFT was also a Registered Trademark pertaining to its promoting and educating of the concept as used within this sector.
Specifically, MID-LIFT refers to the "half-lift" point of motion, of the engine's valve, where all angular dimensions for determining the rocker arm's pivot points are measured to establish a perpendicular relationship on both sides of the rocker arm with the valve and push-rod, respectively. It covers both a design concept of the rocker arm itself, and equally important, the application of that product in use within the internal combustion engine. It is most specifically clear about the definition of what is used to avoid over-arcing and under-arcing.
Other theories of rocker arm design exist, some dating back more than a century, whereby these other variations use different angles of reference taken from different points of valve lift, or no specific valve lift, to determine the overall design with the push-rod and the valve.