The gutter is a single straight line of cells along the axis of symmetry of a mirror-symmetric pattern. Most commonly this is an orthogonal line, and the pattern is then odd-symmetric (as opposed to even-symmetric, where the axis of symmetry follows the boundary between two rows or columns of cells).
The birth rule for Conway's Life trivially implies that if there are no live cells in the gutter of a symmetric pattern, new cells can never be born there since they will always have an even number of living neighbours. For examples of patterns containing empty gutters, see 44P5H2V0, 60P5H2V0, Achim's p4, brain, 274P6H1V0, centinal, p54 shuttle, pufferfish, snail, spider, and pulsar (the latter of which has gutters along all four lines of symmetry).
The edge of a bounded plane without wraparound is equivalent to a gutter. Half of a gutter-symmetric pattern can always be supported by the edge of a plane.
A skew gutter is similar to a gutter, except that one side of the gutter is shifted along the gutter. Skew-gutter-symmetric patterns are known in non-totalistic rules.