Well, it depends, most of the time, the word if flung around in anger, spite or bitterness.
For example, I am arguing with a calvinist
"blah blah blah election election foreordained from the foundation of the world god hates those he didn't die for elect election elected predestinated praise god he chose me blah blah blah"
In my anger, I might say (and have said, to my shame)
"You stupid heretic, you believe God damns billions for His sovereign pleasure."
A more brotherly, loving (agape love of course, it's the bible correctors favourite) way of saying that would be
"You heretic, you believe God damns billions for His sovereign pleasure."
hehe
But more seriously, the word should only be used of those who deny the literal truth of the Bible - 6 day creation, the rapture, the second advent of Christ, but especially the blood atonement, salvation by grace through faith alone, the virgin birth, the miracles of Jesus Christ etc.
In the modern sense of the word, and the way people use it today, a heretick can be saved. For example, a heretick might not believe in the rapture, but still believes in the blood atonement (his doctrine on the end times is heretical, but he himself is not really a heretick). In the biblical sense of the word however, it is one who has completely denied Jesus Christ and is unsaved.