'Terme' comes from "terme lithologique", i.e. the name given to a particular rock type. From there, however, it came to be applied to the physical rock of a given type itself, maybe especially when they don't know which precise 'terme lithographique' applies to the material concerned. So you get 'terme argileux' when they don't know (or care) if the correct 'term' is argilite, argilolite, shale, kaolinite, glauconite, montmorillonite, attapulgite, chlorite, vermiculite, etc. or anything in between.

* "Ainsi, la séquence fondamentale de la Formation ferrifère de Lorraine est caractérisée par une augmentation progressive de la dimension des constituants détritiques de la base vers le haut. Chaque TERME LITHOLOGIQUE défini par une distribution granulometrique se développe avec des figures propres de structures."
* "à chaque profondeur correspond un TERME LITHOLOGIQUE STRICTEMENT DÉFINI. La succession de ces différents TERMES a pu être établie en allant des calcaires compacts marins aux calcaires à Chara lacustres et s'étend sur trois milieux différents: marin, lagunaire et lacustre-continental.
Les alternances des différents TERMES à l'intérieur de cette série que nous venons de définir sont régies par les variations de profondeur, en d'autres termes [a false positive] par les mouvements du fond marin."
* "SÉQUENCE LITHOLOGIQUE : succession évolutive univoque de TERMES LITHOLOGIQUES LIÉS, verticale ou horizontale, présentant des limites naturelles

You could do the same and talk about 'tool' (from 'garage tool') when you're not sure if you're talking about a monkey wrench, adjustable spanner, box spanner, open spanner, hex spanner, torque wrench, etc.

Of minerals, in this sense, you could talk about the 'species', as in:

* "In stratigraphy and tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant CLAY SPECIES is illite. Other common CLAY SPECIES, and sometimes dominant, are montmorillinite and kaolinite."
* "The expansion of basal spacings of montmorillonite - type m inerals with the introduction of water or other polar liquid between layers is a diagnostic feature of many CLAY SPECIES."

Since the French love transposing the abstract into the physical, 'terme' has come to be applied to the structure composed of a particular (or unspecified) 'species', be it a bed several hundred metres thick or dispersed lenses or thin intercalations.

* "Il s’agit de niveaux globalement non argileux pouvant comporter localement un TERME ARGILEUX (INTERLITS pour l’essentiel)."
* "présentant des TERMES ARGILEUX non prédominants et discontinus, par exemple sous forme de POCHES, de LENTILLES ou d'INTERCALATIONS rares."
* "le faciès rubéfié en boule B représente un TERME de passage ENTRE LA ROCHE-MÈRE ET LE FACIÈS ARÉNISÉ rubéfié C ;"
* "De même Wollast [...], Watson [...],[...]décrivent des profils sur roches granitiques dont les TERMES DE BASE, souvent discrets, SONT SURTOUT GIBBSITIQUES : la kaolinite ne semble se former que plus tard, vraisemblablement aux dépens de la gibbsite."
* "a subdivisé cette formation en DEUX TERMES: un TERME INFÉRIEUR DE180M D’ÉPAISSEUR constitué par une alternance de marnes gypseuses et de bancs calcaires. Cette succession est surmontée par une séquence de marne devenant plus carbonatée au sommet. L’ÉPAISSEUR DE CE TERME SUPÉRIEUR EST DE 300 M."

I don't think we have transposed the word the same way in English, though that is indeed what has happened to a certain extent here (where they are talking about actual geological formations):
* "The random mixed-layer clay minerals that represent intermediate structures between two single CLAY SPECIES (e.g., illite and smectite; Brindley and Brown, 1980; Moore and Reynolds, 1989) are labeled as follows (Lucas, 1962): (10−14s) illite-smectite; (10−14v) illite-vermiculite; and (14c−14s) chlorite-smectite."

The above, however, is the work of P. Debrabant,2 M. Lopez,2 and H. Chamley (2)
(2) Sédimentologie et Géodynamique, URA 719 du CNRS, Université de Lille, 59 655 Villenueuve d’Ascq, France.

Bref, you are looking at 'clay deposits' or 'bodies of clay'
*"The CLAY DEPOSITS in the Ostrander Member are lenses or tabular BODIES as much as a few feet thick and several tens of acres in areal extent that are intercalated with ferruginous sands [...] Irregular BODIES OF GRAYISH WHITE CLAY occur within the Ostrander orange sand. Commonly, as at the Thomforde and Clay Bank pits, the clay is completely enclosed by the sand. [...] Aside from the clay deposit in sec. 5, the LARGER BODIES OF CLAY appear to lie on upland surfaces some distance from the thicker deposits of the Iron Hill Member."
*."In thin section the rock appears as a loosely knit mosaic of calcite grains through which are scattered numerous calcitic spherulites with radial structure. Irregular BODIES OF CLAY scattered through the concretion are probably bits of bentonite that became separated by enlargement of calcite grains"
* "CLAY DEPOSITS occur locally on gravel bars (environments 2 and 3) and may be preserved beneath gravel during bar migration."