  The omnipresence of charge density waves (CDWs) across almost all cuprate
families underpins a common organizing principle. However, a longstanding
debate of whether its spatial symmetry is stripe or checkerboard remains
unresolved. While CDWs in lanthanum- and yttrium-based cuprates possess a
stripe symmetry, distinguishing these two scenarios has been challenging for
the short-range CDW in bismuth-based cuprates. Here, we employed
high-resolution resonant inelastic x-ray scattering to uncover the spatial
symmetry of the CDW in Bi$_2$Sr$_{2-x}$La$_{x}$CuO$_{6+\delta}$. Across a wide
range of doping and temperature, anisotropic CDW peaks with elliptical shapes
were found in reciprocal space. Based on Fourier transform analysis of
real-space models, we interpret the results as evidence of unidirectional
charge stripes, hosted by mutually 90$^\circ$-rotated anisotropic domains. Our
work paves the way for a unified symmetry and microscopic description of CDW
order in cuprates.
