How does the psychology of religion literature define the terms religiousness, spirituality, faith, and sacred? Despite the growing interest in and recognition of spirituality and religiousness in psychology, these faith constructs remain poorly defined. We conducted a literature review, comparing existing definitions of these 4 constructs, looking for a consensus, and presenting 4 preliminary working definitions based on the literature. We then undertook 3 studies to find out the prototype phenomena being used by peer-reviewed psychology of religion authors when they define these terms. All 3 studies used definitional content analysis, a procedure based on narrative analysis, grounded theory, and Consensual Qualitative Research. Study 1 examined how 34 articles defined the term spirituality. Study 2 examined how 36 articles defined the term religiousness. Study 3 examined how 471 articles defined the terms religiousness, spirituality, faith, and sacred. The results of these 3 studies indicate that these 4 terms are multidimensional, overlapping, and often poorly defined; nevertheless, the basic elements of their meanings can be discerned. Spirituality, religiousness, faith, and the sacred involve 20, 19, 20, and 9 discernible characteristics, respectively. We propose 4 research-based operational definitions for future research. Specifically, we propose that spirituality is a search for or relationship with the sacred; religiousness is ritual, institutional, or codified spirituality which is culturally sanctioned; faith is a synonym for spirituality and/or religiousness; and the sacred is manifestations of the divine, existential meaningfulness, or an ultimate concern as perceived by an individual.