An Unstable Intermediate (also called the Transition State) is the state of reacting chemicals between their resting state and their final product state. When two chemicals react, their speed depends on several factors; to speed the reaction up, a catalyst may be introduced that quickly brings both chemicals to their unstable intermediate state, which, being unstable, changes to the final product state.
To arrive at the unstable intermediate state, you must first have two chemicals to react. Their reaction may not contain an unstable state, especially if the reaction is slow. When a catalyst is introduced, for example an enzyme to a biological substrate, the reaction time is decreased, and the chemicals pass through the unstable state to reach their final product state quickly. The unstable state is by definition temporary, and cannot normally be maintained. The progression therefore is:
Chemical+Chemical+Catalyst=Unstable Intermediate State=Final Product State