Noun
べっさつ
別冊の付ろくをいつも楽しみにしています。
I am always looking forward to free separate volumes.
かつて、別冊少年ジャンプというざっしがありました。
Once, there was a magazine called "Separate Volume Shonen Jump."
1985年、雑誌Tofuguの別冊本に、生まれたばかりのコウイチの写真が掲載されました。
In 1985, newborn Koichi’s photograph was published in an extra issue of the Tofugu Magazine.
Usually you call books "volumes" when they come in sets, right? This is like that, but separate, making this a separate volume.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The べつ gets shortened to べっ, though, probably because it's a つ and that's what つs do for a living in jukugo words.