1. (Science: chemistry) The linking of the chains of a polymer to one another so that the polymer, as a network, becomes stronger and more resistant to being dissolved.
2. (Science: molecular biology) The abnormal linking of two strands of dna by covalent bonds (as opposed to the normal hydrogen bonds between base pairs), which can occur by exposure to x-rays. Such linking is a type of damage to the dna molecule and must be repaired before the dna can replicate and function properly again.