What does directory permission 'S' mean? (not lower case, but in upper case)

'S' = The directory's setgid bit is set, but the execute bit isn't set.

's' = The directory's setgid bit is set, and the execute bit is set.

SetGID = When another user creates a file or directory under such a setgid directory, the new file or directory will have its group set as the group of the directory's owner, instead of the group of the user who creates it.

To remove the setGID bit:

chmod g-s eclipse/

Taken from man chmod:

You can set or clear the bits with symbolic modes like u+s and g-s, and you can set (but not clear) the bits with a numeric mode.

link to a similar question: https://unix.stackexchange.com/questions/27250/uppercase-s-in-permissions-of-a-folder