An ensemble is a mezcal made from different agaves which go through the entire production process together. They are harvested, roasted, crushed, fermented, and distilled all together. This almost definitely represents the original mezcal when agaves were harvested ripe regardless of what species. The idea of a single agave mezcal most likely is a late 20th century creation inspired by other spirits and wines while also enabled by a more lucrative international market.
This definition of the term “ensemble” reflects a cultural distinction between the traditional way of making mezcal and a contemporary blending after distillation of single agave types. Making a classic ensemble requires a considerable degree of experience because of all the different agave sizes and consistencies. Generally post distillation blends are called mezclas but there is no legal definition of these terms and makers are free to call any mezcal that mixes different agaves by either name or something else. Here is a deeper dive on the entire topic of ensembles and mezclas.