fiducial

The adjective fiducial describes something as being taken as a standard. The term is used in science, e.g., within a description of some new kind of analysis, the term may be used to refer to a normal analysis-method from which this proposed kind departs. It may refer to a model or to particular parameter values typically used with some well-known model. A paper might use the term for something generally taken as a standard, or for something chosen for the purposes of that one paper to be standard, e.g., rather than some specific widely-used parameter values, a selection that is within a range generally considered reasonable. The phrase fiducial model basically means the same as standard model (for models such as the standard model of particle physics, the Lambda-CDM model, or the standard model of a flare).

The term fiducial has some particular usages and meanings regarding particle-physics experiments, but I won't attempt to describe them.

The term fiducial marker (or fiducial mark) has a specific meaning: something included in an image (e.g., photograph) with the purpose of revealing the size of the image's main subject. In a photograph of small objects, it could be a ruler or recognizable coin, or for somewhat larger objects, it could be a person or human form. An astronomical image might be presented including an inserted fiducial mark, or some object of known size within a "raw" image might be said to serve as a fiducial mark.

The term fiducial inference exists in mathematics, but is unlikely to be of interest to astrophysics: it is a past, proposed method of probability inference aimed at capturing the benefits of Bayesian statistics but without the disadvantage of requiring a prior, but it was demonstrated only in cases too simple to attract interest in real use. Mathematicians sometimes discuss the method and variants, but as far as I can tell, this meaning of fiducial is not used within physics.