A curve drawn to pass through the peaks of a graph, such as that of a moduated radio-frequency carrier signal.

The sum of all cell-surface elements that are located outside the plasma membrane.

The glass or metal housing of an electron tube or the glass housing of an incandescent lamp.

1. The envelope of a one- parameter family of curves is a curve which has a common tangent with each member of the family.

2. The envelope of a one-parameter family of surfaces is the surface swept out by the characteristic curves of the family.

The outer membranous lipoprotein coat of certain viruses. Also known as bulb.