nodes and antinodes

On a vibrating guitar string you can find spots that stay perfectly still while, halfway between them, the string is a blur of motion. The still spots are nodes, the wildly moving spots are antinodes. The question it answers is: in a standing wave, which points move and which do not?

A node is a point in a standing wave where the displacement is always zero, the two overlapping waves there are permanently out of step and cancel. An antinode is a point where the displacement swings to a maximum, the waves there stay in step and reinforce. Nodes and antinodes alternate, the distance from one node to the next is half a wavelength (lambda/2), and an antinode sits exactly midway between two nodes.

You can locate them physically: a fixed end of a string must be a node (it cannot move), while a free end is an antinode. In a pipe, a closed end forces a node in the air's motion and an open end an antinode. This is exactly how the geometry of an instrument picks out which notes it can sound. An honest note: for sound, displacement nodes and pressure nodes are opposite, a point of zero air motion is a point of maximum pressure swing, so be clear which quantity you mean.