The Collaborative International Dictionary of English v.0.48:

Layer \Lay"er\, n. [See Lay to cause to lie flat.]
   1. One who, or that which, lays.
      [1913 Webster]

   2. [Prob. a corruption of lair.] That which is laid; a
      stratum; a bed; one thickness, course, or fold laid over
      another; as, a layer of clay or of sand in the earth; a
      layer of bricks, or of plaster; the layers of an onion.
      [1913 Webster]

   3. A shoot or twig of a plant, not detached from the stock,
      laid under ground for growth or propagation.
      [1913 Webster]

   4. An artificial oyster bed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

layer
    n 1: single thickness of usually some homogeneous substance;
         "slices of hard-boiled egg on a bed of spinach" [syn:
         layer, bed]
    2: a relatively thin sheetlike expanse or region lying over or
       under another
    3: an abstract place usually conceived as having depth; "a good
       actor communicates on several levels"; "a simile has at least
       two layers of meaning"; "the mind functions on many strata
       simultaneously" [syn: level, layer, stratum]
    4: a hen that lays eggs
    5: thin structure composed of a single thickness of cells
    v 1: make or form a layer; "layer the different colored sands"

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "layer":
   Appleton layer, F layer, Heaviside-Kennelly layer, Van Allen belt,
   arrange in layers, belt, bookie, chemosphere, delaminate,
   desquamate, exfoliate, flake, ionosphere, isothermal region,
   laminate, lay down, lay up, lower atmosphere, outer atmosphere,
   photosphere, scale, stratify, stratosphere, stratum,
   substratosphere, tropopause, troposphere, upper atmosphere



The Free On-line Dictionary of Computing (30 December 2018):

protocol layer
layer

    The software and/or hardware environment of
   two or more communications devices or computers in which a
   particular network protocol operates.  A network
   connection may be thought of as a set of more or less
   independent protocols, each in a different layer or level.
   The lowest layer governs direct host-to-host communication
   between the hardware at different hosts; the highest consists
   of user application programs.  Each layer uses the layer
   beneath it and provides a service for the layer above.  Each
   networking component hardware or software on one host uses
   protocols appropriate to its layer to communicate with the
   corresponding component (its "peer") on another host.  Such
   layered protocols are sometimes known as peer-to-peer
   protocols.

   The advantages of layered protocols is that the methods of
   passing information from one layer to another are specified
   clearly as part of the protocol suite, and changes within a
   protocol layer are prevented from affecting the other layers.
   This greatly simplifies the task of designing and maintaining
   communication systems.

   Examples of layered protocols are TCP/IP's five layer
   protocol stack and the OSI seven layer model.

   (1997-05-05)