6S is a mnemonic device used to recall the steps to improve workplace organization and effectiveness. It is essentially 5S with safety specifically added.

The 6 "S" Terms are:

Sort
Straighten
Scrub
Standardize
Sustain
Safety

In truth, safety should always be considered when making any changes, observations, or assessments about a workplace. That said, it makes a good deal of sense to use 6S vs. 5S. Unfortunately, though, 5S is deeply ingrained in Lean culture, and would be extremely difficult to supplant with a new term. Regardless of which term you choose to use, though, be sure to make sure safety is an integral part of your workplace organization efforts.