Correct spelling, definition: this noun is a combination of two words: ice and berg (hill). It appeared in the late 18th century and it originates from Dutch ijsberg. We must stick to the etymology when spelling this word. In several Germanic languages (for example German, Dutch, Norwegian, and Swedish [Danish: Bjerg]), berg means mount, mountain, or cliff. There is no reason to write it with a vowel -u. Therefore, the only correct form is iceberg and the incorrect one is iceburg.

Definition of iceberg:
noun, a large floating mass of ice detached from a glacier
The Titanic hit an iceberg and sank.
The ship struck a hidden iceberg.

Idioms with iceberg:
Some most commonly used idioms include: the tip of the iceberg (a small part of a larger problem).
I think you should know that her today’s absence from school is just the tip of the iceberg.

Incorrect spelling, definition: the noun iceberg is a combination of two elements, neither of which should be changed. The word berg is widely known in many languages and means mountain. Therefore, this word cannot be spelled with the letter -u instead of -e. We have to follow the etymology. That’s why iceburg is the wrong form and iceberg is the correct one.