We have identified the impurity defect responsible for the trigonal paramagnetic signal (T defect, g∥=1.989 and g⊥=1.852) and yellow coloration observed whenever a cubic zirconia sample (yttrium or calcium stabilized) undergoes chemical reduction or irradiation treatments. We have prepared single crystals of yttria-doped zirconia with small controlled amounts of titanium impurity ranging from 0.01 to 0.1 at. %. After reduction a perfect correlation between the intensity of the T-defect signal and Ti concentration is obtained, which rules out previous models based on the assumption of an intrinsic origin for that defect. Rather, T defects are due to Ti3+ impurity ions which are also responsible for other paramagnetic signals in the same g<2 range with symmetry lower than trigonal. The g⊥ value of the T defect and its linewidth vary with Y2O3 concentration. Some defect models are tentatively proposed.