In many pieces, the composer will seemingly shift to a different key for a long period of time, rather than just temporarily. While tonicization involves temporary treating a chord as tonic, modulation involves an actual change of tonic. For this reason, some people colloquially refer to modulations as "key changes." That term, however, is somewhat problematic for two reasons. First, most of the time, the key signature itself does not change. Instead, accidentals are used while maintaining the original key signature. For the second reason, consider for example a shift from C major to C minor. The key signature changes, but the tonic scale degree (C) is the same. Still, we will refer to the keys before and after a modulation as "the original key" and "the new key," respectively.

Modulations are most convincing when they do one or more of the following:

Predominant, dominant, tonic progression resulting in a cadence in the new key
Cadential 6/4 in the new key
Continuing in the new key after a cadence

Otherwise, you can argue that an event is a tonicization or extended tonicization (involving three or more chords) rather than a modulation.