A domain is a category of language or content that shares vocabulary, style, and subject matter. A medical drama is a different domain than a cooking show, which is a different domain than a fantasy novel, which is a different domain than a casual conversation between friends.
Beyond the most common 2000 words, the vocabulary you need depends fairly heavily on what you're consuming. The next 1000 most common words in a medical drama are completely different from the next 1000 in a sports podcast. If you spread your immersion across many different domains, you'll encounter lots of words but each one less often — which makes them harder to learn.
By focusing on a smaller number of domains at first, you encounter the same specialized vocabulary repeatedly, which means you learn it faster. Research suggests that with focused domain immersion, you can reach 90%+ comprehension of a specific type of content with far fewer words than you'd need for broad comprehension across all content types.
Phase 1-2: When you're still a beginner, the only domain that's really accessible to you will be "Comprehensible Input" and maybe childrens TV. That's perfectly fine to use in this phase. Or if you're the type of person who needs to use more interesting content, picking 1 easier domain is best.
Phase 2B-2C: In this pahse, your goal is to 
Phase 3+: As your level increases, you can broaden into more domains. Each new domain requires learning some new vocabulary, but the process gets faster because vocabulary starts to overlap a lot more.
Phase 6-7: Deliberately seeking out unfamiliar domains is how you fill vocabulary gaps and round out your fluency.
Pick content you genuinely enjoy. The best domain for learning is the one that makes you want to keep immersing. Some good starting domains:
There are lots of things to choose from, so don't worry too much about making the perfect choice or sticking with this list. However, some good things to avoid as a beginner are domains like comedy (too reliant on cultural knowledge) and current events (also relies on culteral knowledge, plus uses more formal language that doesn't overlap as much with day to day conversation).
Domains aren't isolated. A lot of vocabulary is shared across domains, and mastering one domain makes the next one easier, especially after the 3rd or 4th. The jump from domain 3 to domain 4 is much smaller than the jump to domain 1 from nothing.
Nation (2006) demonstrated that the vocabulary needed for comprehension varies significantly depending on the type of text. His analyses showed that a single novel requires far fewer word families for 98% coverage than a broad corpus of mixed texts, because vocabulary recycles more within a consistent genre. Sutarsyah, Nation, and Kennedy (1994) found a similar pattern when analyzing a single university economics textbook — learners needed roughly 5,000 word families to reach adequate coverage of that one domain, compared to the 8,000–9,000 needed for general written English.
Webb and Rodgers (2009) extended this finding to television, showing that different TV genres have distinct vocabulary profiles. The specialized vocabulary within a genre — the words beyond the most frequent 2,000–3,000 — concentrates heavily, meaning that sticking with one type of show produces far more repeated encounters with the same words than jumping between genres. This matters because repeated encounters are one of the strongest predictors of vocabulary acquisition. Webb (2007) found that the more times a learner encounters a word, the more dimensions of knowledge they acquire — not just recognition but also collocational and grammatical knowledge.