Okay, here's Websters definition. We all know he is the authoritative source for the English language. As you can see, even Webster's definition can be vague. He talks about "precisely half", then goes on to say "partial or partly", definitely not precisely half.
The term Semi-scale in our hobby is like a generic term, like kleenex. It doesn't really mean any particular thing except that the train is not true to prototype.
Definition of semi-
1
a : precisely half of: (1) : forming a bisection of <semidiameter> (2) : being a usually vertically bisected form of (a specified architectural feature) <semidome>
b : half in quantity or value : half of or occurring halfway through a specified period of time <semiannual> <semimonthly> — compare bi-
2
: to some extent : partly : incompletely <semicivilized> <semi-independent> <semidry> — compare demi-, hemi-
3
a : partial : incomplete <semiconsciousness> <semidarkness>
b : having some of the characteristics of <semiporcelain>
c : quasi- <semigovernmental> <semimonastic>