defective verb, only second-person present imperative, no subjunctive or indicative

  1. Look [at this]! See [this]!
  2. Take heed, listen 
  3. Watch and learn. 
  4. Pay attention [to someone or something specified]!
  5. Focus! Concentrate!
  6. (World Wide Web) Click [here], check [this out]
  7. Come (a request or invitation to move toward the speaker)

from iej, take notice of, observe