Unique listeners show how many different accounts listened to the song, in other words, unique listeners don't show stream but the number of different accounts listening to that song. Unique listeners' lists are usually made based on daily or total amount of the listeners. (*Those who listen to a song as soon as they are released are fans waiting for the song to release, so the first hour list of unique listeners're generally belongs to the groups have lbig fandom.) Melon stopped sharing data on the total number of unique listeners a few years ago. Therefore, only daily and hourly data can now be seen. You can see the songs that have reached the most daily unique listeners so far down below:

Source:Melon