An alternate is something or someone that serves in place of another. An alternative is a second option that does not replace the first. For example, when a road undergoing maintenance is closed to traffic, you have to take an alternate route.

Alternate and alternative are easy to confuse. 'Alternate' means a replacement for the original (i.e., the original is not an option). 'Alternative' means another option to the original (i.e., the original is still on option).

Usage commentators have often emphasized that 'alternate' should be used to mean "occurring or succeeding by turns" and 'alternative' to mean "offering or expressing a choice." But in many cases where choice is involved, 'alternate' is a viable alternative.

Alternative and alternate can both be adjectives or nouns. Alternate means every other one in a series; e.g., if something happens on alternate days, nights, etc. it happens on one day, etc. but not on the next.