  Spontaneous symmetry breaking constitutes a paradigmatic classification
scheme of matter. However, broken symmetry also entails domain degeneracy that
often impedes identification of novel low symmetry states. In quantum matter,
this is additionally complicated by competing intertwined symmetry breaking
orders. A prime example is that of unconventional superconductivity and
density-wave orders in doped cuprates in which their respective symmetry
relation remains a key question. Using uniaxial pressure as a domain-selective
stimulus in combination with x-ray diffraction, we unambiguously reveal that
the fundamental symmetry of the charge order in the prototypical cuprate
La$_{1.88}$Sr$_{0.12}$CuO$_4$ is characterized by uniaxial stripes. We further
demonstrate the direct competition of this stripe order with unconventional
superconductivity via magnetic field tuning. The stripy nature of the
charge-density-wave state established by our study is a prerequisite for the
existence of a superconducting pair-density-wave -- a theoretical proposal that
clarifies the interrelation of intertwined quantum phases in unconventional
superconductors -- and paves the way for its high-temperature realization.
