alternate / alternative
Taking turns or every other, versus another option.
Use alternate as a verb for taking turns (the two chairs alternate), as an adjective for every other (alternate Saturdays), and as a noun for a substitute person (an alternate on the jury). Use alternative for another choice: an alternative plan, no alternative. Alternate reality and alternative reality both appear; alternate reality is the usual set phrase. Do not use alternate when you only mean “other.”
Examples
UseWe meet on alternate Fridays.
UseThey offered an alternative route.
AvoidThey offered an alternate route. (Acceptable if you mean a substitute already designated; not if you only mean another option.)
UseThe novel is set in an alternate reality.
In some technical and official prose, alternate is used for any backup option. That use is established. In general use, keep alternative for the choice and alternate for rotation or a named substitute. The above is our guidance.