Enclosure is the more general, catch-all term that avoids the debate entirely, and it's increasingly the word used in care guidance because it doesn't imply a specific style of build. A glass-fronted wooden cabinet, a mesh-sided setup for arboreal species, and a fully planted bioactive tank could all reasonably be called an enclosure, regardless of whether someone else in the hobby would call them a vivarium or a terrarium.
For more detail, see
Reptile Centre UK.
This matters practically for a few reasons. Substrate that's too fine or too coarse for a species' natural digging pattern can cause problems that generic mixes don't always account for. Humidity retention that suits a rainforest species will behave very differently under a desert-adapted animal's heat lamp. A range built around one species removes some of that guesswork, since the substrate, decor and accessories are designed to work as a set rather than assembled from separate, unrelated products.
Most reptile substrates are sold as general-purpose products, suitable in theory for a wide range of species with only minor adjustments. The ProRep Beardie Life range takes a different approach, building a substrate and accessory system around a single species, the bearded dragon, rather than adapting a generic mix to fit.
Strictly speaking, a terrarium originally referred to a planted enclosure built around live vegetation, closer to a small indoor garden than a housing unit for an animal. A vivarium, by contrast, technically refers to any enclosure built to house a living creature, whether that's a reptile, amphibian or invertebrate, with or without live planting. In practice, though, UK retailers and keepers use both terms fairly loosely, often depending on regional habit or supplier naming rather than any strict definition.
The structure is fairly simple in practice. A subscription is set up around a delivery cadence, typically weekly or fortnightly, chosen to match how quickly a particular animal or collection gets through its livefood. Each delivery contains a set quantity of feeder insects, sized appropriately to the animal being fed, whether that's smaller insects for a juvenile or hatchling, or larger prey for an adult with a bigger appetite. The livefood itself is despatched to arrive live and in good condition, with appropriate packaging for the time of year and the journey involved.