An endospore is a structure that is temporarily dormant and non-reporductive and is produced by the bacteria belonging to the Firmicute phyllum. It is called an endospore because it is suggestive of a spore but not totally because it is of a seedlike form which may also mean it comes from within.
Endospores are formed intacellulary and are eventually released from the mother cell as a free cell. Endospores are the most durable cell found in nature and can remain viable for very long periods of time. http://textbookofbacteriology.net/Bacillus_3.html