Penny sleeve
A penny sleeve is a thin, soft, clear plastic sleeve for one card, used as a first layer of protection before the card goes into a toploader, a semi-rigid holder or a binder.
In more detail
BCW sells its version as Standard Card Sleeves and says they are often called penny sleeves. They are acid-free polypropylene, measure 2 5/8 by 3 5/8 inches for a standard 2 1/2 by 3 1/2 inch card, and fit inside a standard 3 by 4 inch toploader.
Ultra PRO lists its Penny Sleeves at 45 microns front and back, against 50 microns for its premium sleeves and 90 microns at the front of its art sleeves. The thin film keeps off dust and fingerprints, but it adds no stiffness: Cardboard Gold notes that a semi-rigid holder gives much more protection than a soft sleeve.
Sleeve the card before it goes into a harder holder. Cardboard Gold suggests a penny sleeve inside a Card Saver I for a more secure fit and inside a toploader for extra protection, and both CGC and TAG ask you to put each card in a sleeve and then in a semi-rigid holder when you send cards to them.
Related terms
Where Rip.fan covers it
Updated