What is the meaning of "transition area" as used by the FAA? Has the meaning changed over time since the term was introduced?
As per the FAA Procedures for Handling Airspace Matters:

Transitional areas, Class E, are designated to serve terminal and en route aircraft to include helicopter operations such as:
a. Transitioning to/from terminal and en route.
b. Transiting between airways and routes.
c. En route climbs or descents.
d. Holding.
e. Radar vectors.
f. Providing for course changes.
g. When the route under consideration is almost all within existing Class E airspace and small additions would complete the coverage.
NOTE-
The only areas that are normally excluded in the Class E description should be limited to Mexico, Canada, SUA and international airspace. Exclude SUA only when active. Do not exclude Federal Airways or other airspace areas.
h. En route training operations.

Link here.
I don't know that this definition has necessarily &quot;changed&quot;, but they apparently thought it needed clarification in 2011.