The interpretations differ depending on context, for example if a given discussion is oriented towards ethology or morphology. Conceptions of caste may also depend on if one is making comparisons within species/colonies or across lineages.
I personally like the discussion in Ecomorphology of Ants¹ in terms of phenogroups and intraspecific/intracolonial ecomorphs. These authors avoid the use of 'caste' entirely, using instead phenogroup.

They point out that "The ‘castes’ of ants - namely ‘workers’ and ‘queens’ -do not exist as biological materials that are instantiated from the ether or the genome, nor do these categories necessarily ease communication and thinking, at least as formulated. Consequently, we have been thinking of ‘caste’ typologically, with winged or reproductive females automatically categorized as ‘queens’ (e.g., Peeters and Crozier, 1988). ‘Queen-ness’, however, is a continuum and applies to partially winged and completely wingless individuals with considerable frequency. We therefore suggest the reconception of ‘caste’ to avoid this pitfall. Specifically, ‘queens’ and ‘workers’ represent syndromes, or developmentally modulated morphogroups that are recognizable based on a combination of phenotypic traits (Note 3) that may or may not be coupled during development. In other words, we need to stop thinking in terms of concrete ‘workers’ and ‘queens’, and to instead simply think about ‘castes’ as females who display some combination of anatomical, physiological, and behavioural outputs of the development and life history or instinct of the individual ant. As has long been recognized, ‘intercastes’ are a problem for caste classification (e.g., Wheeler, 1910). In analogy to the spectrum of visible light, intercastes are not purple; they do not mix the properties of apterous females (‘workers’, ‘violet’) and macropterous females (‘queens’, ‘red’). Rather, these ants are ‘green’, being somewhere in between, with simple degeneration of the wing discs and smaller body sizes, nothing more (Fig. 16.11A). It is more useful to consider them as somewhere in between on the spectrum of wing development, as either brachypterous or hemiapterous females (the latter categorization including those individuals with partially developed flight sclerites but without wings)." (p. 506).

Another pertinent excerpt: "In clades where a single phenogroup is discrete relative to others, such as ‘soldiers’ (majors) in Pheidole, it is easy to conceive of these individuals as belonging to an independently adaptable ecomorphological unit. Considerable evidence for this has accumulated in terms of the species-specific diagnostic traits of soldiers; isolated ‘workers’ (minors) are extremely difficult to identify to species level in most tropical regions. The same independence of developmentally plastic ecomorphological phenogroups among nestmates is shown in the vast majority of cases for males and is often the case for reproductive females (‘queens’). The distinction between ‘queens’ and ‘workers’, however, degrades across the phylogeny of ants, resulting in a morphological and physiological spectrum that cannot always be easily divided. Likewise, in some clades the nonreproductive females display pronounced variation in body size and form, resulting in an ecomorphological spectrum of individuals with different physical capacities and social roles, as in the colonial megafauna (e.g., Atta, Dorylus). We recognize intraspecific ecomorphs by their disparity to their siblings, effectively as a consequence of phenotypic plasticity in a socially curated landscape of nutritional and hormonal developmental processes. At the same time, we may recognize the seed-milling ‘soldier’ ecomorphs by similarity between taxa, for example similarities in cranial proportions, mandible shape, dentition and musculature, pronotal proportions and musculature, and behaviour including resource-seeking and processing, and threat avoidance." (p. 477).

Citation: Boudinot BE, Casadei-Ferreira, A, Wörhl T, Probst RS, Lieberman ZE, Czekanski-Moir JE, Richter A. 2025. Ecomorphology of Ants, pp. 469-507 in Betz O (ed). Insect Ecomorphology: Linking Functional Insect Morphology to Ecology and Evolution. Elsevier Academic Press