What Happened
Patent claims have to be clear enough that someone skilled in the field can understand what's covered and what isn't. That's the "definiteness" requirement. If claims are too vague, they're invalid. The Supreme Court set the standard in a 2014 case called Nautilus: claims must inform those skilled in the art about the scope of the invention with "reasonable certainty."
Why It Matters
This sounds straightforward, but it's not. What's "reasonably certain" to one person might be hopelessly vague to another. Terms like "about," "approximately," "substantially," and "near" are common in patents but can be problematic. So can functional language that describes what something does without explaining how it works.
The Federal Circuit has been wrestling with this for years. Some terms are fine in context—if the specification explains what "substantially parallel" means, or if someone skilled in the art would understand it from the technical field. Other terms fail because they have no clear boundary.
The stakes are high. An indefinite claim can't be enforced, and it can't be fixed after the patent issues (except through reissue, which is complicated). For accused infringers, indefiniteness is a powerful defense that can knock out a patent without needing to find prior art.
What You Should Know
If you're drafting patents, be as specific as possible. Define potentially ambiguous terms in the specification. Use numerical ranges instead of relative terms when you can. Make sure functional claims are tied to concrete structures or algorithms.
If you're enforcing patents, review your claims for potential indefiniteness before suing. If you're defending, look for vague terms that could support an indefiniteness challenge.
The bottom line: clarity matters. Vague patents don't serve anyone—not inventors, not competitors, not the public.