What does "seed" mean in cryptography? I have come across "seed" many times, and at each time, it confuses me more and more. I think it has a well-known meaning in cryptography, so what is it?


  They can usually use the  limited number of results stemming from a
  limited number of seed values to defeat security.
  
  Not only does complex manipulation not help you if you have a limited 
  range of seeds, but blindly-chosen complex manipulation can destroy 
  the entropy in a good seed!


Those are some parts of an article I was reading, I understand the context, but I'm not sure about "seed".
The seed of a pseudorandom number generator — whether cryptographically secure of not — is the initial input that defines the pseudorandom sequence of outputs generated from it.
It's not really a term that's specific to cryptography, except insofar as there's a considerable amount of overlap between pseudorandom number generation and cryptography, which might be why you haven't been able to find a good definition of it in crypto literature.
That said, such definitions do exist.  For example, NIST SP 800-90A rev. 1 provides the following, admittedly perhaps not the most easily digestible one:

Seed
Noun : A string of bits that is used as input to a DRBG mechanism. The seed will determine a portion of the internal state of the DRBG, and its entropy must be sufficient to support the security strength of the DRBG.
Verb : To acquire bits with sufficient entropy for the desired security strength. These bits will be used as input to a DRBG mechanism to determine a portion of the initial internal state.
Also see reseed.

(Note: &quot;DRBG&quot; stands for &quot;Deterministic Random Bit Generator&quot;, which is essentially the NIST term for a pseudorandom number generator.)
SP 800-90A also includes a more detailed description of seeds in section 8.6 (appropriately titled &quot;Seeds&quot;), which I would strongly recommend taking a look at if you're interested in PRNG seeding in a cryptographic context.  It may, however, be a good idea to start reading a few pages up from section 7 (&quot;Functional Model of a DRBG&quot;) to familiarize yourself with the concepts and terminology first.