s/s

souvenir sheet.

fdc 

first day cover: the stamp, sheet, or souvenir sheet are adhered to an envelope and postmarked the day, city, and country of issue.

perf

perforation: this refers to the little teeth around the circumference of the stamp.

imperf

imperforated: this refers to the lack of perforations around the circumference of the stamp.

bklt

booklet of stamps.

og

original gum: this means that the gum on the stamp(s), sheets, or souvenir sheet(s) is original as issued.

nh

never hinged.

h

hinged.

h/r

hinge remnant: part of the hinge is stuck to the um.

thin

part of the stamp is missing paper: this can be seen as a translucency when the item is placed in watermark fluid or held to a strong light.