tailor's chalk

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A thin piece of hard chalk used in tailoring for making temporary alteration marks on clothing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A chalk-like material used for marking alterations on fabric in tailoring and dressmaking. The marks are brushed or washed off the material when no longer needed.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun chalk used by tailors to make temporary marks on cloth