What is a collective dismissal? And Its legal ground?

It is the termination of the contract affecting at least two or five employees, depending on whether the company has between 2 to 50 or more than 50 employees, and this simultaneous termination has to be within three months.

The reasons for those terminations have to be at least one of the following:

• Definitive closure of the company; Bankruptcy
  
• Closure of one or more departments
  
• Reduction in staff for structural, technological or market reasons