分断前部 refers to an orphan (the first line of a paragraph left at the bottom of a page), while 分断後部 is a widow (the last line(s) carried over to the next page).
Compound of 分断 (division, break) + 後部 (rear part, latter part), literally 'divided latter part'. The exact derivation as a typesetting term is uncertain; it is a rare technical coinage.