Rami Arieli: "The Laser Adventure" Chapter 10 Section 1, page 6
Definition of Coherence Length
The maximum path length difference between the two laser beams in which they still performs interference is called: ”Coherence Length (Lc)”.
The maximum time delay corresponding to this path difference Lc/c is called “Coherence Time”:
tc = Lc/c
Attention !
These definitions relate to temporal coherence and not to spatial coherence.

Using Michelson Interferometer it is easy to measure coherence length by measuring the maximum path difference between the two beams, which still show the interference pattern.
There are certain gas lasers which have very long coherence length of tens of meters, while other lasers, especially Diode lasers have coherence length of the order of millimeters.

In holography, which is based on interference between light beams (as explained later), long coherence length enable taking holograms of large bodies which require long depth of field.

Both the light reflected from the near part of the body, and the light reflected from the far part of the body, will still be coherent with the reference beam.

Although there is a large path difference between the two beams, this length is less than the laser coherence length.