spearhead

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The sharpened head of a spear.
• noun The leading forces in a military thrust.
• noun The driving force in a given action, endeavor, or movement.
• transitive verb To be the leader of (a movement, for example).

from The Century Dictionary.

• noun The head of a spear.

from the GNU version of the Collaborative International Dictionary of English.

• noun The pointed head, or end, of a spear.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The pointed head, or end, of a spear.
• noun One who leads or initiates an activity (such as an attack or a campaign).
• noun The leading military unit in an attack.
• noun sports A player who initiates attacking moves
• verb transitive To drive or campaign ardently for, as an effort, project, etc.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who leads or initiates an activity (attack or campaign etc.)
• noun the head and sharpened point of a spear
• verb be the leader of
• noun the leading military unit in an attack