Tonicization & Modulation
Tonicization involves entering a new key briefly, but not permanent. In contrast to modulation, there is no authentic cadence (perfect cadence ) during a tonicization that can induce a permanent change of key.
Example of a tonicization:
Modulation is the process of changing keys, and key must be changed by using an authentic cadence (perfect cadence )
Example of a modulation:
You should keep in mind that in tonal music, the mains degrees are I, IV and V.
In this example, The beginning is in C Major and next (measure 3 and 4) we have some chords issued from the A minor key, and at the end of this score you will find an authentic cadence (perfect cadence ). This cadence induces the change of key, so there is a modulation.