Employee engagement is a psychological state, and the construct is contested. Macey and Schneider's 2008 review states that "The meaning of employee engagement is ambiguous among both academic researchers and among practitioners who use it in conversations with clients" [3]. Schaufeli's review adds that Shuck's search of 213 eligible publications produced four separate definitional approaches [2], so two organizations can both say they measure engagement while measuring different things.
Kahn's personal engagement, 1990
The first scholarly article on engagement at work was published by William Kahn in the Academy of Management Journal in 1990 [2]. Kahn defined personal engagement as people employing and expressing themselves "physically, cognitively, emotionally, and mentally during role performances" [2], and Albrecht, Furlong, and Leiter summarize his three enabling conditions in Frontiers in Psychology as "psychologically meaningful work, psychological safety, and the availability of physical, emotional, and psychological resources" [4]. Those conditions are properties of the workplace rather than fixed traits of the person, which the same authors describe as "context specific, labile, and amenable to change" [4].
The UWES measures the state
Most academic engagement research uses the Utrecht Work Engagement Scale, which operationalizes the vigor, dedication, and absorption definition [2]. It began as 24 items, was cut to 17 after psychometric evaluation, and now exists in 17, 15, and 9 item versions with internal consistency usually between .80 and .90 [1].
Gallup's Q12 measures the conditions
Gallup defines employee engagement as "the involvement and enthusiasm of employees in both their work and workplace" and measures it with the Q12, twelve agreement items following a single overall satisfaction item, administered to nearly 64 million employees since 1998 [6][5]. What the instrument captures matters here: the items ask about role clarity, resources, fit, feedback, and feeling appreciated, and Gallup calls the set "a formative measure of 'engagement conditions,' each of which is a contributor to engagement through the measure of its causes" [5]. The most widely used engagement survey in the world is, on its author's own description, substantially a measure of the employee experience.