Single Issue
A single issue, also called a floppy, is one thin stapled installment of a comic series, usually around twenty to thirty-two pages, published monthly.
Where the word comes from
"Floppy" is collectors' slang for the soft, stapled magazine format, as opposed to the bound spine of a book.
In depth
American comics are serialized in single issues: saddle-stitched pamphlets of roughly twenty story pages plus covers and advertising, sold in specialist comic shops. Each issue is numbered, and most arcs run five or six issues before being collected in a trade paperback.
Variants and printings
Single issues are often released with several covers — variant covers by different artists — and successful issues may be reprinted as second or third printings. These matter to collectors; the story inside is the same.
Reading in issues or in books
Reading in issues means reading the story as it comes out, a chapter at a time. Reading in collections means waiting, then reading a whole arc at once. Bookshops mostly carry the collections.
Comic shops
Single issues are sold mainly through specialist comic shops, which order them in advance from distributors. Many readers keep a pull list at their shop: a standing order for the series they follow. Bookshops and online retailers concentrate on collected editions instead.
Collecting
Single issues are the collectible form of comics: first appearances of characters, first printings and variant covers can become valuable. For reading, collected editions are cheaper and easier to store.
What it means today
If you want a series now and in full, start with the collected editions. Single issues suit readers following a series month by month.