tremendous

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

• adjective Extremely large in amount, extent, or size; enormous: synonym: enormous.
• adjective Very great in scope or importance.
• adjective Remarkable; outstanding.
• adjective Archaic Capable of making one tremble; terrible.

from The Century Dictionary.

• Such as may or does excite trembling, fear, or awe; overpowering in character or quality; awful; dreadful: as, a tremendous explosion; tremendous invective.
• Hence Such as to excite astonishment or awe; unexampled; wonderful in a high degree; overwhelming; astounding: used intensively or hyperbolically.
• Synonyms Frightful, terrific, horrible, appalling.

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

• adjective Fitted to excite fear or terror; such as may astonish or terrify by its magnitude, force, or violence; terrible; dreadful

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

• adjective awe-inspiring; terrific.
• adjective Notable for its size, power, or excellence.
• adjective Extremely large (in amount, extent, degree, etc.) or great

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

• adjective extreme in degree or extent or amount or impact
• adjective extraordinarily good or great ; used especially as intensifiers
• adjective extraordinarily large in size or extent or amount or power or degree