Tonicization in music theory is often interchanged with the term modulation. While both are based on the concept of changing key within a composition to create dramatic interest, the main differentiating factor is duration. Tonicizations tend to be brief and based on movement by leading tone or tonic/dominant chord progression. Harmonically, when a chord in one key is used as the dominant of another in order to change keys, it’s called a “secondary dominant” or a “pivot” chord. For example, in the key of C, an F major chord is the IV-chord, however it can be thought of as the V-chord of Bb major, and as such, used to establish Bb as a temporary new tonal center. Melodically, any scale degree or accidental can be used as a leading tone to move to a new tonal center. For example, in the key of C, the note G can be used as a leading tone to the key of Ab, which is related to the key of C.
A tonicization is considered a modulation when any sense of the original key is lost, or the accidentals of the tonicization remain for longer than 10 seconds or so. Another condition of a modulation as opposed to tonicization is when the new tonal center has a distinct or authentic cadence.