An alternating knot diagram is a knot diagram in which the crossings alternate between "over" and "under" as you travel along the knot. An alternating knot is a knot which has an alternating diagram.

For example, the diagram for the trefoil knot 3_{1} below is alternating, whereas the diagram for the knot 8_{19} is not alternating.

3_{1} |
8_{19} |

[1] Lickorish, W. B. R., *An introduction to knot theory*. New York: Springer (1997).