The "Crossover" title can be applied to any metal genre crossing into an outside genre, but it is most commonly used to describe the mixing of Hardcore Punk and Heavy Metal along the lines of early DRI.
The title "Thrash" was the original name given to this Crossover genre from its punk roots.
Before this, early hardcore bands such as Black Flag were named/described as 'Thrash' in their local circles but as Hardcore emerged as a genre the bands were grouped in their place as Hardcore Punk. A few years later "Crossover Thrash" emerged, this genre took the name previously used to describe skating and also the fast and hard sound of early Hardcore.
Soon after the new genre of metal "Speed Metal", as it was first known, emerged. It was named so because of its fast tempo and swift style of riffery. No-one called it 'Thrash Metal' untill the mid-to-late 80s. Though some lyrics of Speed Metal bands may contain the word "thrash", it is in reference to the mosh pit and headbanging.
Here is an excerpt from a recent interview with the Speed Metal band Sacrifice who formed in '84 and released their debut in '85 (on a side note: they were fairly influential to Death Metal, along the same lines as Possesed)
How did the idea to form a Speed Metal band with Joe come up exactly? I mean it's not because you both liked Metal that you had to form a band, know what I mean?
Rob: “There were no bands here playing the new style of Metal which wasn’t even called Thrash yet back then… we called it Speed Metal. Joe lived down the street from me and we both liked the same kinds of bands so we decided to find a bassist and drummer. We weren’t really thinking about being rock stars or anything, we were still just learning how to play back in 82 / 83 or so. It was a simple idea to begin with.”
-End Quote http://www.sacrifice.com.br/interview_01.asp
Also I would like to point out the stupidity of renaming a genre even though the previous name suited fine and then new name is alread taken by another genre.
When 'Thrash Metal' is counted by fools as a genre seperate to Speed Metal there is a lot of blurred lines as to bands that fit into each. With plain Heavy Metal or Hair Metal bands filling the void of Speed Metal such as Judas Priest and Twisted Sister.
Ignore the masses: its not "Thrash Metal" its SPEED METAL