What is the definition of crossing over / recommendation?

Answer: It is exchanging genetic material between two paired chromosomes

Crossing over is the exchanging of genetic material between two paired chromosomes. Crossing over is a way to recombine the genetic material so that each person (except for identical twins) is genetically unique. It is essential for the normal segregation of chromosomes during meiosis. Crossing over also accounts for genetic variation, because due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical.

Explanation:

We are learning the same things, apparently. I just learned this myself, and am working on it right now.