  The Ensemble of trajectories $x(0 \leq t \leq T)$ produced by the Markov
generator $M$ can be considered as 'Canonical' for the following reasons : (C1)
the probability of the trajectory $x(0 \leq t \leq T)$ can be rewritten as the
exponential of a linear combination of its relevant empirical time-averaged
observables $E_n$, where the coefficients involving the Markov generator are
their fixed conjugate parameters; (C2) the large deviations properties of these
empirical observables $E_n$ for large $T$ are governed by the explicit rate
function $I^{[2.5]}_M (E_.) $ at Level 2.5, while in the thermodynamic limit
$T=+\infty$, they concentrate on their typical values $E_n^{typ[M]}$ determined
by the Markov generator $M$. This concentration property in the thermodynamic
limit $T=+\infty$ suggests to introduce the notion of the 'Microcanonical
Ensemble' at Level 2.5 for stochastic trajectories $x(0 \leq t \leq T)$, where
all the relevant empirical variables $E_n$ are fixed to some values $E^*_n$ and
cannot fluctuate anymore for finite $T$. The goal of the present paper is to
discuss its main properties : (MC1) when the long trajectory $x(0 \leq t \leq
T) $ belongs the Microcanonical Ensemble with the fixed empirical observables
$E_n^*$, the statistics of its subtrajectory $x(0 \leq t \leq \tau) $ for $1
\ll \tau \ll T $ is governed by the Canonical Ensemble associated to the Markov
generator $M^*$ that would make the empirical observables $E_n^*$ typical ;
(MC2) in the Microcanonical Ensemble, the central role is played by the number
$\Omega^{[2.5]}_T(E^*_.) $ of stochastic trajectories of duration $T$ with the
given empirical observables $E^*_n$, and by the corresponding explicit
Boltzmann entropy $S^{[2.5]}( E^*_. ) = [\ln \Omega^{[2.5]}_T(E^*_.)]/T $. This
general framework is applied to continuous-time Markov Jump processes and to
discrete-time Markov chains with illustrative examples.
