There’s a lot of slop in how the term is applied but it is as others here have described. Some species show different characteristics in populations that inhabit different environments. Salmoniform fishes include a number of species that display so-called phenotypic plastictity, they look different in different environments. A very interesting and important study on Arctic char of Lake Thingvallavatn in Iceland identified four “incipient species” adapted to different modes of existence, two benthics forms with different feeding specializations reflected in their morphology and two pelagic forms with different feeding specializations reflected in their morphology.
Northern cisco (Coregonus artedi) were once abundant in the Laurentian Great Lakes of North America, and exhibited different forms in different places. They were called lake herring by the first English-speaking Europeans who saw them. A number of other cisco species were described, with the individual lakes exhibiting different combinations of types. Eventually, environmental degradation, fishing pressure, invasive species and (as a range of experts observed) genetic introgression from interbreeding eliminated some types entirely and other types from some lakes until it was believed that there were only lake herring/northern cisco and three other species scattered among the lakes. Then blackfin cisco were found in Lake Nipigon. Then in some small lakes in Algonquin Park. Then, by the organization I work for, in a lake between the Algonquin Park lakes and Lake Huron. Genetic analysis told us that these fish were not really very distant at all from northern cisco.
A study by biologists at Laval University found that the diverse cisco “species” had arisen many times in a bunch of lakes and that in every lake the blackfin cisco, short-jawed cisco, kiyi, bloater, short-nosed cisco and whatnot were most closely related to the northern cisco in their own range, not to supposed conspecifics in other lakes.
These days the tendency is to look at these forms as ecotypes or, more specifically, ecomorphotypes. Here’s a blackfin cisco from near where I live. It’s entered in iNat as a northern cisco.
Modern biology is constructed, in large measure, from ideas developed in the 19th Century to explain the existence of species. The fact of species existence in more or less the Biblical model reflected in the Eden and ark stories was an assumption. The more we dig into it the more it becomes apparent that the species concept is a slippery thing with a lot of examples that don’t fit the assumptions.