A Holliday junction is a structure which occurs during homologous recombination between homologous chromosomes. While the two chromosomes are side by side, one strand of recombinant DNA on each chromosome is broken and then attached to the broken strand of DNA on the other chromosome. The crossover point (the Holliday junction) is able to slide up and down between the two chromosomes, so that a little or a lot of DNA can ultimately be switched between them.
This structure was discovered in 1964 and named after researcher Robin Holliday.
Some of the information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.