1. A mirror between windows.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• oier glass,
• lier glass,
• -ier glass,
• 0ier glass,
• puer glass,
• pjer glass,
• pker glass,
• poer glass,
• p9er glass,
• p8er glass,
• piwr glass,
• pisr glass,
• pidr glass,
• pirr glass,
• pi4r glass,
• pi3r glass,
• piee glass,
• pied glass,
• pief glass,
• piet glass

The adjective form of the phrase "pier glass" is "pier-glassed".

The plural form of "pier glass" is "pier glasses"

The singular form of the word "pier glass" is still "pier glass". It does not change in its singular form.