The ‘whether’ and ‘if’ I’ll look up today is the conjunctions. Merriam-Webster’s definitions:
1. a:in the event thatb:allowing thatc:on the assumption thatd:on condition that
The rest of the definitions are not related to the usage of whether.
1. until the early 19th century a direct question involving alternatives
2. an indirect question involving stated or implied alternatives
3. alternative conditions or possibilities
Ergo, the two words do share a meaning in certain places. In general, both ‘if’ and ‘whether’ can be used to indicate alternatives, while ‘if’ is used to indicate possibilities. Also, when any of the two can be used, ‘whether’ is more appropriate for formal writing.
If – Indicating possibility or alternative. (e.g. If I get there early, I’ll grab a beer ; I wonder if he’s going to do it)
Whether – Indicating an alternative, usually in choice. (e.g. I wonder whether he’s going to do it)