In music, tonicization is the treatment of a pitch other than the overall tonic as a temporary tonic in a composition. Tonicization is achieved through the use of the scale and harmonies of the tonicized key. The most common method of tonicization uses leading tones, dominant-tonic chord progressions, or a combination thereof. Tonicization is an example of tonal chromaticism. Tonicization is not very different from modulation, because (according to Schenker),"there's no such thing as a modulation in music", because a modulation is nothing more than a long tonicization, in which the influence of the previous key is still evident.
A tonicized chord is a chord to which a secondary dominant progresses. For example ii in V/ii. Only major and minor chords may be tonicized. Though perceptions vary  as a general rule if a chord is treated as the tonic for longer than a phrase then the treatment is considered a modulation.
Change of scale 
To achieve a change of tonic, the notes that define a key (that is, the scale) must be changed. For example, the C major scale consists of the notes C-D-E-F-G-A-B-C. If a tonicization of G major is desired, F must change to F-sharp to fit into the G major scale: G-A-B-C-D-E-F♯-G. Thus, to tonicize G major, F-sharp (as the leading tone to G) is used in place of the F-natural of the original tonic C major scale.
Similarly, to tonicize F major, the B of the C major scale must be changed to B-flat to produce the F major scale: F-G-A-B♭-C-D-E-F. B-flat functions here as a downward-pushing leading tone that resolves to A as the third of the F major triad.
These examples illustrate the most common altered tones used in tonicization: scale steps 7 → 8 (the traditional "leading tone," such as F♯ → G in G major) and scale steps 4 → 3 (such as B♭ → A in F major) in the tonicized key. Especially when briefly tonicizing more remote keys, the alteration of one or both of these scale steps may be sufficient, instead of the significant modifications needed to change an entire scale to the scale of a remote key.
Use of secondary harmonies 
Introducing altered tones melodically often produces only a weak feeling of tonicization. Stronger tonicizations are frequently achieved by borrowing not only pitches from the tonicized key, but also chords (known as "secondary chords" or "secondary harmonies"). The most common such chord is the secondary dominant; this is simply a chord that is dominant to the tonicized key and is usually one of the following: V, V7, viio (usually not in root position), or viio7 (often in root position). In musical analysis (see diatonic functions), a secondary dominant is notated with a slash separating the tonicized scale degree and the type of secondary dominant chord used. For example, if the original tonic is C Major and a tonicization of F Major (the subdominant and 4th scale degree of C Major) is desired, one could use the V7 chord of F Major (which is C7) as a secondary dominant to approach F. In this case, one could notate the secondary dominant chord with a slash like so: "V7/IV" (pronounced "five seven over four"). If the viio7 of F (Eo7) is used instead of the V7 chord, the notation would like this: "viio7/IV" (pronounced "seven diminished seven over four"). Note that three of the secondary dominant chords mentioned (V7, viio, and viio7) include both the leading tones on scale steps 7 and 4 in the tonicized key.
Longer tonicizations may include other secondary chords, such as a subdominant or the tonic triad borrowed from the tonicized key. Generally the secondary dominant resolves to the tonicized tonic triad, but occasionally this is not necessary to achieve tonicization. Such tonicization without actually sounding the destination tonic is especially common when using a half cadence or deceptive cadence progression. For example, if the overall tonic is C major, a Phrygian cadence moving from F major to E major, as well as a deceptive cadence that moves D minor to E7 to F major, could be heard as tonicizing A minor. (Depending on the context, the use of E major here could also be described simply as a "borrowed" or "altered chord," rather than a tonicization.)
For further details on the varieties of harmonic progressions that can be used to move between keys, see modulation.
For examples of secondary harmonies used in popular music, see List of songs with chromatic harmony