The word "heretic" is inflammatory, and I quite dislike the habit some ardent and otherwise correct defenders of Orthodoxy have of throwing it around loosely. It's actually a highly technical word, and most of the time, other words such as "heterodox" or "erroneous" would be both more accurate and more charitable.
In Orthodox parlance, a heretic is:
a former member of the holy Orthodox Church (for the Church judges no one outside of herself)
who not only believes,
but also teaches what is contrary to Church doctrine
defiantly (viz., after adequate admonition)
and who, as a result, has been excommunicated
and anathematized by the Church
specifically and formally (not just "in effect").
And we perhaps should add, anathematized by the Church in council. (Anathema: something declared to be set aside or put away, permanently consigned to God.)
Merely holding or teaching error does not make a person a heretic; if he was never Orthodox then he is heterodox, or if he repents when admonished, he is Orthodox. A saint's opinion or a patriarch's opinion that someone is a heretic does not make that person one; the whole Church formally and specifically identifying her wayward child a heretic does make him one.
The only "loophole" I can think of that might justify any other usage of the word, "heretic," is the fact that St. Paul has issued some more generic anathemas:
"If any man love not the Lord Jesus Christ, let him be anathema." (I Corinthians 16:22)
"But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be anathema." (Galatians 1:8)
By the first of these sayings, many and perhaps most of us stand more or less under the anathema, so let us by very chary of throwing that word around, as well.
P.S.) Declining to call someone a heretic, or his teachings heretical, does not imply his errors are any the less serious! Heterodoxy can be every bit as pernicious as heresy.