Orchestral build is a term used in disco music to describe the systematic overlapping of prerecorded elements of the symphony orchestra during an interlude of a song.
On the song's break, the melody was reintroduced on top of the drums, timpani, and percussions, by sequential highlighting of the various rhythm instruments, such as the bass guitar, guitar, rhythm guitar, piano, harp, then followed by the woodwinds, then horns, then strings, until all were assembled into a harmonious whole.
As the identifying trademark of the disco mix, the orchestral build was implemented after a break in order to accentuate the melodic structure of the composition, and thereby propel dance-floor participants into a natural-high.
References
Orchestral build Wikipedia(Text) CC BY-SA