A lens of longer focal length than standard.
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Have you ever wondered why people look extremely ugly in passport photos (particularly the ones taken in photo booths)?
The reason is that the picture has been taken from short-range using a wide-angle lens, causing the subject's ears to disappear behind the cheekbones.
To photograph a face in proper proportion, the camera must be a reasonable distance away from the subject (not possible in a photo-booth) and a lens of somewhat longer focal length than a normal (standard) view lens is required in order for the head and shoulders to fill the frame.
For 35mm cameras, the best focal length for portraits is in the range 80 - 135mm, and lenses with focal lengths in this region are known as 'portrait lenses'.