A
parallel shift in the
yield curve is a shift in the yield curve - that is, a change in yields - in which all
maturities change by the same number of
basis points.
In other words, if the yield on the 3 month
T-bill increases 100 basis points
(or one percent), then the 6 month, 1 year, 5 year, 10 year, 20 year, and 30 year rates will all increase by 100 basis points as well.
Some
academic studies has estimated that over 90% of all shifts of the yield curve are indeed parallel.
See also: Non-parallel shift in the yield curve