Researchers often use the terms ESM and EMA interchangeably, referring to studies where survey data (and other types of data) are collected on multiple occasions within the day and over time. However, there are also subtle, if not substantial, differences when we examine the historical motivations behind ESM and EMA.
One of the goals of psychology historically has been to advance the understanding of people in their everyday contexts. Donald Fiske (1971) described how one of the major goals of psychology is "to measure ... the ways a person usually behaves, the regularities in perceptions, feelings, and actions"
Out of this motivation to capture representative activities and experiences, ESM was developed. One of the first applications of this method was on an adolescent sample by Mihaly Csikzentmihalyi, Reed Larson, and Suzanne Prescott. Interestingly, the goal was to understand "What do adolescents do all day long?", "What motivates them to engage in these activities?", and "What are their psychological responses to these activities?"
ESM grew out of this tradition and the focus has primarily been on representativeness of activities and experiences in a population of interest in their natural environments.
As developed by Arthur Stone and colleagues, EMA developed later and grew from the tradition of clinical and health psychology. In the former, this was motivated by behavior therapy and self-monitoring, where the goal was to have participants actively monitor their a specific set of behaviors in a repeated fashion. This included aspects such as addictive behaviors (e.g., smoking) or dysfunctional behaviors (e.g., conflict) in order to address them. In the latter, it was inspired by ambulatory assessments within health settings (e.g., blood pressure monitoring).
Therefore, while ESM focuses on representativeness, EMA focuses on the dynamic unfolding of behaviors in natural environments.
At Expimetrics, we use the term ESM for the historical flavor, reflecting early studies seeking to capture repeated survey data within participants using the aid of technology. However, we use this to refer to all types of ESM and EMA studies, and broadly different types of longitudinal studies.
To aid researchers, I am providing a summary Table seen above which organizes and delineates the differences between ESM and EMA.