Not admitting decomposition; that cannot be decomposed.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. representing the furthest possible extent of analysis or division into parts
Word Usage
"It seems to me that this reasoning rests on an unscientific use of the term _element_; it rests on giving to that class-name the meaning, _substances asserted to be undecomposable_."