Formal:
* Exclusively: This is a close synonym with a similar meaning to "solely."
* Only: Similar to "solely" but slightly more informal.
* Merely: Emphasizes that nothing else is involved, but can have a slightly negative connotation.
* Strictly: Indicates that something is limited to a specific area or purpose.
* Entirely: This implies that something is complete and without any other components.
* Totally: Similar to "entirely" but more informal.
* Purely: Emphasizes that something is without any other factors or influences.
Informal:
* Just: Similar to "only" but more informal.
* Simply: Emphasizes that something is uncomplicated or basic.
* Only and simply: A more emphatic combination of the two words.
* By itself: This emphasizes the singular nature of something.
Other Options:
* By itself: This emphasizes the singular nature of something.
* Independently: This implies that something is not dependent on anything else.
* Unilaterally: This implies that something is done by one person or group without any other input.
The best synonym for "solely" will depend on the specific context of your sentence.