If you look at job sites (like Bayt) you see as many different definitions as there are business analyst jobs advertised, varying from accountant to software architect.
So the tasks and responsibilities of a business analyst may vary widely, depending largely on the department / business you are working for.
In my opinion however, there is one key role that defines business analysis: Translating business requirements into IT models, and so bridging the gap between business and IT architects.
Business analysts typically take the lead role in:
Assisting with the business case.
Planning and monitoring.
Eliciting requirements.
Requirements organization.
Translating and simplifying requirements.
Requirements management and communication.
Requirements analysis.
