Sample characterized by a (unique) sample axis Z with all directions perpendicular to Z being equivalent. In other words, the sample properties are invariant to rotation around Z. Uniaxiality exists in many anisotropic samples and simplifies the interpretation of their spectra considerably.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 437 (https://doi.org/10.1351/pac200779030293)