Terroir is how a particular region's regionality (typically the climate, soils and terrain) affect the taste of a spirit.
It’s an overused term at the best of times, and it’s debateable just how appropriate it is for distilled spirits given how much the process of distilling, the apparatus used to do so and the people influence the end product.
In this context, many spirit’s “terroir” is derived not by the terrain or the soil, but by the distilling traditions of a region, the history of a category and if an aged product – the climate.
Some spirits have more evident 'terroir' than others, such as Mezcal and Eau De Vie, where all these factors are palpable. Their extensive use of native yeasts and wild, open air fermentations (which play a huge role in shaping the flavour) mean that there’s also a good argument that could be made around how bacteria and microbes should also be counted as being a part of a spirit’s regionality too.