Mark Spahn mspahn at LOCALNET.COM
Tue Feb 7 23:20:00 UTC 2006

Asking Google to "define:spike" produces, among other definitions,
     spike = a sharp rise followed [immediately] by a sharp decline.

In my opinion, this is correct, except that the spike on a
quantity-versus-time graph need not be upward-pointing;
it could also be a downward spike (i.e., a sharp decrease
followed immediately by a sharp increase).
I think "immediately" is necessary for this definition,
to ensure that "spike" denotes a spike-like  _/\_-shaped graph
rather than a plateau or mesa that rises and falls abruptly but
remains at a constant level for a long time.

The December 2003 Scientific American (page 35)
has a citation that illustrates this meaning of "spike"
perfectly, in the sense of both an upward-pointing spike 
and a downward-pointing spike.  In the description 
of what a sonic boom is,

     A supersonic jet forms a shock wave at its nose,
     which claps back together after its tail.  This pressure
     suddenly spikes a couple of pounds per square foot
     over ambient atmospheric pressure, then shoots
     below ambient by about an equal amount, spiking
     again before returning to ambient pressure.  A graph
     of pressure over time would form the letter N.

But "spike" seems also (and more often) to be used
simply in the sense of "increase" or "sudden increase",
with no connotation of a decrease soon after the increase.
I have heard news reports of "a spike in gasoline prices".
Another illustration is on page 30 of the same magazine;
discussing the Daubert decision to limit the use of "junk 
science" expert testimony in trials,

     The biggest spike occurred between July 1996 and
     June 1997, when the rate at which science evidence
     was excluded rose to 70 percent, from about 51 percent

Most dictionaries seem to ignore both the "sudden up-then-down
(or down-then-up) movement" meaning and the "sudden increase"
meaning of "spike".

-- Mark Spahn  (West Seneca, NY)

The American Dialect Society - http://www.americandialect.org

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