A pointed homotopy theory is left semistable if Omega Sigma is isomorphic t
o the identity. This gives the proper characterization of "infinite loop sp
aces". It reduces in the standard case to any of the current descriptions,
while the free semistabilization has a universal property which guarantees
its uniqueness. (C) 2000 Elsevier Science B.V. Ail rights reserved. MSG: 18
E; 18G; 55P.