Occurs in a period or larger formal unit when a harmonic motion from the tonic and back to it is interrupted by an inconclusive cadence (half, or somewhat less commonly imperfect authentic) at the end of the first phrase or section, starts over and is completed in the second phrase or section. Consider mm. 1-8 of the theme to Mozart's Sonata in A, K. 331, I as an example.
The music itself is given at (a). Note how the half cadence of m. 4, which comes to rest melodically on ^2 (B natural) is answered by the PAC of m. 8, which moves past ^2 to ^1 (A natural). Comparison of mm. 1-3 with mm. 5-7 shows that after the half cadence of m. 4, which seems to cut short the music's drive to melodic and harmonic closure, the music "starts over," pushing on to the PAC of m. 8.
A reductive analysis, showing only the most structurally important notes in the outer voices, is given at (b). Here it is even clearer that mm. 1-4 are a kind of incomplete version of the completed musical thought of mm. 5-8. The highest notes of the soprano line reveal a stepwise descent from sol (^5) to do (^1) interrupted at m. 4, started over at m. 5 and completed in m. 8. We anticipate closure in m. 4; by postponing that closure until m. 8, a musically pleasing tension between what we expect and what we get is introduced and then resolved.
Interruption is essential to the generation of tonal forms from the simplest parallel period to lengthy sonata forms. If we consider the exposition of a major-key sonata form as the antecedent phrase of a sort of extra-large period, the cadence on V at he end of the exposition serves the role of a structural interruption, the development serves to embellish the interrupting dominant, and the recapitulation serves as the consequent phrase, resolving the structural tensions introduced by the interruption at the end of the exposition and intensified by the development. (Minor-key sonata form movements require a slight adjustment to this model, but still can be understood in terms of it.)