On center: The distance from the center of one board's width to the center of another's. Abbreviated o.c.
Pitch: The angle at which a roof rises, expressed, for example, as a rise of one foot in 12 feet ... or a 1-in-12 pitch.
Plate: A horizontal (or nearly so) board or boards lying at the top or bottom (sole) of a stud wall.
Pilaster: A reinforcing column tied into a masonry wall.
Stud: A vertical member in a framed (or stud) wall ... usually a 2 X 4 or 2 X 6.
Rough opening: The spacing of an opening before trim boards are added.
Header: A horizontal member (of double or triple boards or metal) that transfers the weight above a door or window frame to the adjoining full-height wall members. Or, a member which transfers the load from joists around an opening in a floor.
Trimmer: A stud that runs full height in a door or window rough opening. Jack studs (see below) are nailed to it.
Bird's-mouth: A notch cut across a rafter so that it fits snug on a sill or plate.
Sill: Similar to a plate, this board usually lies on top of a masonry foundation or wall.
Cross rafter: Special term made up to describe one of the short support rafters near the center of a house. Might also be described as a header.
Jack: Any rafter or stud that does not run full length. It is also used to describe what carpenters sometimes call a "cripple".
Miter: Cut at an angle ... assumed to be 45 degrees if no specific angle is mentioned.
Ledger: A strip of wood nailed to a main member, from which notched intersecting members can be hung for temporary support during nailing.
Bridging: Boards nailed between main members to prevent them from twisting and to distribute the load evenly.