The term "Arctic air" is often used casually to describe the very cold weather we sometimes get in winter. However, it really is a reference to air which picked up most of its characteristics near or north of 66.33 degrees north latitude, where there is very little sunlight during winter and it often gets extremely cold. This region includes the Arctic Ocean and a small part of northern Alaska, northern Canada, northern Scandinavia and northern Russia. This is very far north.
It is not unusual in our climate to get a direct flow of air from the Arctic region, and this often results in brutally cold weather. However, our weather can get very cold even when the air hasn't come from the true Arctic. Sometimes, air that's been hovering over Saskatchewan or Manitoba can be well below zero and drop into the Dakotas and Minnesota. It's fine to call this Arctic air as a casual term, but it might not actually be from the Arctic.