Whilst Midgardsormr is right about the "Solo" being the boss, it stands apart from the "roles" mobs are given, as you can get a "Solo Brute".

In 4e you get 4 monster types:

Minions - Like a standard mob, but killed with 1 hit
Standard - Normal mob, normal powers
Elite - A tougher than normal monster/enemy that is usually the strongest in a group (so the Orc Leader who leads the tribe of orcs)
Solo - Typically a standalone mob that is meant to, as the name suggests, fight solo (eg. a Dragon, Sith Lord, or any encounter where combat can still be fun with only 1 target)

In the 4e DM guidebook that Midgardsormr mentions they do actually refer to Elites and Solo's as "Elite and solo monsters represent the toughest foes the characters can face... They make great villains, stars of a campaign, or intimidating "boss" monsters at the climax of an adventure".

Basically, anyone who is going to be overly picky about the term they use should just be ignored; some people will find fault in anything