: We sought to characterize the variance and overlap among definitions of Proximal Junctional Kyphosis (PJK) used in the adult spinal deformity (ASD) literature. PJK is defined as excess in PJK angle, a Cobb angle between the upper-instrumented vertebra (UIV) and a supra-adjacent vertebra (SAV), either one (UIV+1) or two (UIV+2) levels rostral of the UIV. No expert consensus exists for threshold angle or which SAV to use. : A total of 116 thoracolumbar fusion patients ≥ 65 years old were reviewed. The UIV+1 and UIV+2 angles were measured. Six definitions of PJK from the literature were evaluated. These definitions were selected based on citation frequency, historical relevance, and accessibility through commonly used databases. Pearson's Chi-squared and pairwise comparisons were performed to evaluate the distinctness and agreement rates among these definitions. : The six definitions of PJK were as follows: PJK angle ≥ 20° with UIV+2 as the (SAV), PJK angle ≥ 10° with a >10° change from pre-op with UIV+2 as the SAV, PJK angle > 2 standard deviations from average with UIV+1 as the SAV, PJK angle ≥ 10° with a >10° change from pre-op with UIV+1 as the SAV, PJK angle > 15° with UIV+1 as the SAV, and PJK angle > 30° with UIV+2 as the SAV, or displaced rod fracture, or reoperation within 2 years for junctional failure, pseudoarthrosis, or rod fracture. [PJK10] and were the most distinct definitions while , , , and showed no significant pairwise differences. [PJK2SD] was stringent, while definition included unique diagnostic information not captured by other definitions. : The use of , , , or is recommended for consistency, with presenting the best balance. Stringent may be beneficial for identifying severe PJK, though with low sensitivity. Overall, PJK definitions must be standardized for the consistent reporting of clinical outcomes and research comparability.