General synonyms:
* Commitment
* Devotion
* Loyalty
* Zealousness
* Enthusiasm
* Passion
* Perseverance
* Assiduity
* Diligence
* Fervor
* Ardor
More specific synonyms:
* For a task: Assiduousness, Industry, Application
* For a person: Affection, Love, Admiration
* For a cause: Conviction, Faithfulness, Selflessness
The best synonym will depend on the specific meaning you want to convey.