Blended learning has become a popular alternative to traditional instruction. Professional development that supports teachers’ practice of the phenomenon continues to evolve (Shand & Glassett Farrelly, 2017). The purpose of this phenomenological, mixed method study was to investigate high school teachers’ perceptions, through the lens of Knowles’ adult learning theory (Knowles, Holton, & Swanson, 2015), of the definition of blended learning, the impact previous professional development had in shaping definitional understanding and implementation of blended learning, and perceptions of future professional development needs. Few studies have focused on secondary blended learning professional development and the impact shared definitions of blended learning had on the effectiveness of professional development (Gurley, 2018; Halverson, Spring, Huyett, Henrie, & Graham, 2017; Shand & Glassett Farrelly, 2017). Analysis of quantitative data, collected in four Southwest Missouri high schools, revealed emergent definitional themes that informed the development of the qualitative instrument. Responses from 12 teacher interviews were examined and four themes emerged: interpretations, technocentric, instructional backing, and professionals’ needs. Findings revealed a shared definition of blended learning did not exist among teachers. Teachers perceived previous blended learning professional development was ineffective. Teachers perceived future blended learning professional development that defined and modeled blended learning, and acknowledged their needs as adult learners, would support their practice of blended learning. The conclusions reached in this study have important implications for blended learning professional development planners and instructional support specialists.