A glacier is ice, formed by layer upon layer of snow that begins to flow under its own weight. It may end on land, in which case it can form the headwaters of a river or (as in the dry valleys of Antarctica) simply sublime into very dry air. It begins, however, in an area that is glaciated, or covered with compacting snow. That area may be a small as a glacial cirque or as large as Antarctica.
Continental-scale glaciations today are limited to Antarctica and Greenland. But 18,000 years ago, much of North America to south of the Great Lakes and Eurasia into the Alps and Carpathians was covered by a solid sheet of ice. The Himalayan glaciation was also much more extensive than is the case today, and the Rockies were also covered with ice. In fact, all mountain glaciations were more extensive then, including the Brooks and Alaska Ranges in Alaska.
Interestingly, while New York State, the Great Lakes, and northern Europe were covered with mile-thick ice, interior Alaska and large parts of northern Asia remained ice free, a cold steppe that supported mammoths, long-horned bison, and horses. The Bering Sea was mostly land, due to lower sea level, and many scientists believe that the first inhabitants of North America moved from Siberia to Alaska with no idea that they were entering a new continent. But there is no evidence that there has ever been glaciation where I live, in Interior Alaska. It was and is too dry. Only the frozen bones of long-extinct animals preserved in permafrost are left to tell the tale.