Formal:
* Well-informed: This is a general term for someone who has a lot of knowledge about a particular topic.
* Knowledgeable: This synonym emphasizes the breadth and depth of the person's knowledge.
* Erudite: This suggests a person with deep and extensive knowledge, often gained through academic study.
* Savvy: This implies that the person is not only knowledgeable but also has a good understanding of how to use that knowledge to their advantage.
Informal:
* In the loop: This means that the person is kept up-to-date on all the latest information.
* Up-to-speed: This suggests that the person is aware of current events and trends.
* In the know: This is a common phrase that means someone is aware of what is happening.
* Connected: This suggests that the person has a network of contacts that provide them with inside information.
* Wired: This is a slang term for someone who is always in the know about the latest news and gossip.
Other:
* Insider: This term is used for someone who has access to confidential information.
* Guru: This implies that the person is an expert in a particular field.
* Oracle: This suggests that the person has a special ability to predict future events.
The best synonym to use will depend on the context of your writing.