Complement

Definition :

A word or word group that completes the predicate in a sentence.

'complement' is also used in a wider sense. We often need to add something to a verb, noun, or adjective to complete its meaning. If somebody says I want, we expect to hear what he or she wants; the words the need obviously don't make sense alone; after hearing I'm interested, we may need to be told what the speaker is interested in. Words and expressions which 'complete' the meaning of a verb, noun, or adjective are also called 'complements.'

• I want a drink, and then I want to go home.
• Does she understand the need for secrecy?
• I'm interested in learning to fly

• A subject complement follows a linking verb; it is normally an adjective or a noun that renames or defines in some way the subject.
  • A glacier is a huge body of ice.
  • Glaciers are beautiful and potentially dangerous at the same time.
  • This glacier is not yet fully formed. (verb form acting as an adjective, a participle)
  Adjective complements are also called predicate adjectives; noun complements are also called predicate nouns or predicate nominatives. See predicates, above.
• An object complement follows and modifies or refers to a direct object. It can be a noun or adjective or any word acting as a noun or adjective.
  • The convention named Dogbreath Vice President to keep him happy. (The noun "Vice President" complements the direct object "Dogbreath"; the adjective "happy" complements the object "him.")
  • The clown got the children too excited. (The participle "excited" complements the object "children.")
• A verb complement is a direct or indirect object of a verb. (See above.)
  • Granny left Raoul all her money. (Both "money" [the direct object] and "Raoul" [the indirect object] are said to be the verb complements of this sentence.) 
  
  
  
  
  

source :
http://grammar.about.com/od/c/g/complterm.htm
http://grammar.ccc.commnet.edu/grammar/objects.htm