The incorrectly spelled word is:

☀ ecstacy

The correct spelling is "ecstasy". It refers to an overwhelming feeling of joy, happiness, or intense emotion.

Explanation of the other options:

☀ athlete: This word is spelled correctly. It refers to a person who participates in sports or physical exercises professionally or competitively.

☀ diagnosis: This word is spelled correctly. It refers to the identification or determination of a medical condition or disease.

☀ accumulate: This word is spelled correctly. It means to gather or collect over time.