Formal:
* Obscure: Difficult to understand because it's unclear or hidden.
* Abstruse: Difficult to understand because it's complex or highly technical.
* Arcane: Difficult to understand because it's mysterious, secret, or known only to a few.
* Inscrutable: Impossible to understand or interpret.
* Cryptic: Difficult to understand because it's mysterious or ambiguous.
* Esoteric: Difficult to understand because it's limited to a small group of people with special knowledge.
Informal:
* Confusing: Difficult to understand because it's unclear or misleading.
* Baffling: Difficult to understand because it's perplexing or puzzling.
* Bewildering: Difficult to understand because it's overwhelming or disorienting.
* Unintelligible: Impossible to understand because it's unclear or jumbled.
* Muddy: Difficult to understand because it's vague or unclear.
* Jargon: Difficult to understand because it's specialized language used by a particular group.
The best word to use depends on the specific context. For example, you might use "obscure" to describe a text that's difficult to understand because it's written in an old or unusual style. You might use "baffling" to describe a puzzle that's difficult to solve.