From the Oxford English Dictionary:

A pair of spectacles kept in position by a spring which clips the nose, rather than by earpieces. Cf. nose glasses n.

A larger image of what Pince-Nez would typically look like:

A round pair of glasses that have no ear pieces at the sides, but a spring like mechanism to hold the glasses on the nose, and an additional component for a chain to the lefthand side.

Photo courtesy of iStock

Editor of the Strand, Herbert Greenhough Smith, famously wore Pince-Nez himself.

An illustration of Smith sitting at a desk, facing the viewer. He is wearing a suit and his signature Pince-Nez glasses

Herbert Greenhough Smith