Here is my take at the moment ;)
Possibly widening the breadth a little to encompass things such as scat, fur, feathers, other remains, tracks, and etc:
An observation is the evidence of the presence of life preferably recorded in the form of an image or sound with the specification of time and place.
A stimulus resulted in that record. That record can just be that unto itself or can have a purpose or give a result on its own or in combination with other records.
The stimulus is multifactorial and can be impulsive, planned, automated, reactive, academic, non-academic, and etc.
The purpose and/or result is/are also multifactorial with additional evidence of behavior/interaction, habitat, growth, individual, difference, scale, morphology, rerproduction, range, persistance, posture, gregariousness, group dynamic, visibility, blend/camouflage , diet, checklist, and etc.
Because of the multifactorial nature, observations are difficult to define from one person to the next and from one moment to the next.