Can be used in two slightly different ways in our field but in both cases it’s synonymous with “value of/for a variable (for person x at time t)”. The more restricted use tend to denote something quite literally observed by an observer: a rating or perhaps a reading from a scales, blood test or scan; however the term is also used for any value of/for a variable. It’s probably a bit of a hang over from when data collected/observed by professionals were given much higher status than self-ratings.
Try also #
Runs throughout the book really.