In biology what is crossing over?

Crossing over is the process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other.

Crossing over is exchange of genetic material between non sister chromatids of homologous chromosomes during meiosis. This would result in new allelic combination in the daughter cells. Crossing over between chromatids occurs in prophase I of meiosis I. It exactly occurs in the pachytene stage where recombination nodules forms.