The monolingual transition is the shift from using bilingual dictionaries (target language → native language) to monolingual (also known as "normal") dictionaries (target language → target language).
This forces your brain to think about words in terms of the target language rather than translating to your native language.
Bilingual dictionaries create a mental shortcut: every target-language word gets mapped to a native-language equivalent. But many words don't translate perfectly, and this mapping can distort your understanding. A monolingual dictionary defines words using the target language itself, which helps you build a more native-like understanding.
This isn't really a problem for languages that are quite related. English and Spanish, despite looking pretty different, have very similar ways of making words. Bilingual dictionaries are perfectly functional for reaching a very high level. Only occasional words (usually slang) require a native language definition. And some dictionaries don't even contain these words.
For less related languages (English and Japanese, for example), it makes more sense. Many basic Japanese words are hard to translate into English. You might see a list of 10-15 English words used to describe a single Japanese word.
The biggest problem is that dictionaries are verbose and technical. When you're trying to learn a language, that can be incredibly frustrating.
For closely related languages (Cousin/Similar): consider starting the transition toward the end of Phase 2B, once you can read basic definitions. However, it's not required. Plenty of people have reached high-level fluency without them.
For distant languages: wait until Phase 2C or even later into Phase 3. Monolingual definitions in distant languages require substantially more vocabulary to understand, despite being more potentially useful. Bilingual dictionaries aren't bad and will make your beginner-intermediate phases much easier.
Another option that's much easier than a full on monolingual dictionary is asking an AI chatbot. They're often very good at giving simplified explanations of words without being overly technical. Try asking something like this:
Can you please explain "this word or phrase" to me in simple Italian?
The transition is gradual, not sudden: