Being a Senior developer, when your given a new title of Lead developer. is a lead a promotion?

Look at your pay check.

When it comes to developers, the titles can mean completely different things depending on the company and team size. IMO, a lead SHOULD already be a Sr, but not vice-versa. Unless the LEAD is just in charge of managing/assigning tasks.

Usually. Lead developers are practically a more hands-on director. They have a lot of say over the teams technical direction and can usually enforce certain practices that others should follow. However, please note you are then the go-to teacher as well. Once I became a lead software architect, I spent more time teaching people and sitting in meetings than actually coding.

lead here. it differs per company what a "lead"is.

In my opinion a lead developer's primary task is to lift the team as a whole to a higher level.

Besides that, you're a damn good developer, have communicative skills towards clients and pm, and are involved with putting the right man on the job.