  Fermionic pairing and the superfluid order parameter change dramatically in
low-dimensional systems such as high-T$_c$ superconductors. Here we show how
the order parameter dynamics, which defines essential collective properties, is
modified by strong confinement. Using a model system for strongly correlated
superfluidity, an ultracold fermionic gas, we study the response to a weak
modulation of the confinement. Surprisingly, we observe a well-defined
collective mode throughout the entire crossover from the
Bardeen-Cooper-Schrieffer (BCS) state to Bose-Einstein condensation (BEC) of
molecules. Starting in the BCS regime, the excitation energy follows twice the
pairing gap, then drops below it in the strongly correlated regime, and finally
approaches twice the harmonic level spacing imposed by the confinement in the
BEC regime. Its spectral weight vanishes when approaching the superfluid
critical temperature. The experimental results are in excellent agreement with
an effective field theory, providing strong evidence that amplitude
oscillations of the order parameter hybridize with and eventually transform
into spatial excitations along the confined direction. The strong modification
of the excitation spectrum highlights the relevance of confinement to fermionic
superfluids and superconductors, and raises questions about its influence on
other fundamental quantities.
