What exactly is severance pay?

My husband overheard a conversation in December in which his boss was offering his job to another employee. When confronted his boss told him yes he was planning on laying him off. My husband asked him to please get with him ASAP on the details of his departure. Needless to say this went on for almost 7 weeks with his boss "dodging" the discussion. We both feel like the "dodging" was becuase he was caught off guard and not prepared for my husbands departure with a replacement. At the end of February he finally told him that beginning March 1st he should start taking a couple of hours out of each working day to work a job search and he would get back with him on more details. Today he has received a written document in which his boss states that his last day of employment is April 30th. He also stated that he is receiving 10 months of severance pay but since he told him March 1st to begin his job search that March and April are considered the first two months of his severance pay and his last month would be December. My husband has taken very little time off at his job for a job search because of his loyalty to his team and he also did not want to jeapordize any chances of a good severance package so he has been working very hard at his job and earning every bit of his paycheck. We know that severance is not required and that chances are great that he will only get paid through December but I personally don't like the implication that his boss has made that he is getting 10 months severance. Is there any reason we should argue with his semantics? Or do you think we should just let this go. I guess one question would be is how is "severance pay" actually defined?