the point in a song at which time (not tempo) slows down. it is usually associated with metal, hardcore, metalcore, and other sub-genres of metal and hardcore music. often the guitar during a breakdown is palm-muted and is played using breaks or rests to create a choppy, brutal sound. the guitar can also be played strummed or rung out and still be considered a breakdown. what makes the part of a song a breakdown is mostly determined by the speed of the drums and also somewhat the style in which the drummer plays in that speed.
(band name) - (song title): sparing no lives - painting the murder, lamb of god - pariah, terror - overcome, the red chord - antman, unearth - endless, animosity - 24 more, iniquity - desiderated profligacy. all of the songs above have good examples of breakdowns in my opinion. i strongly suggest the first song on the list.