The word "heresy" comes from the Greek word αἵρεσις, meaning "choice", which over time came to take on a negative tone associated with particular schools of thought revolving around
chosen opinions advanced by particular leaders of said schools. Thus it has been popular since ancient times that heresies be named after their supposed leaders: Nestorianism after those who followed the teachings of Nestorius, Pelagianism after the followers of Pelagius, Origenism after the followers of Origen, and so forth. The charge against my own communion was sometimes called (and still is in some corners, often heavily represented on the internet among the MOST TRUE ORTHODOX™ EVER, SEVEN COUNCIL CRUCIBLE ANTI-ECUMENIST type of Chalcedonian
) Eutychianism, after Eutyches, a presbyter and archimandrite at Constantinople who advanced a confused and heretical Christology that claimed that Christ's divinity in some sense 'consumed' his humanity "like a drop of vinegar in the ocean" (as the quote goes), as that is what the believers in the dyophysitism as expressed in the Tome of Leo took us to believe in instead of their own chosen opinion. In return, in addition to believing the Chalcedonians to be heretics and crypto-Nestorians, the term "Melkite" -- now used in a self-identifying, value neutral sense by Arabized Greek Catholics of the Levant -- was originally coined as a pejorative in the Syriac language by opponents to Chalcedon who saw their Syriac bretheren who accepted the council as having turned their backs on Orthodoxy in favor of following the imperial church and court into heterodoxy (from Syr. ܡܠܟܝܐ
malkoyo 'royal, imperial').
But obviously Eastern Orthodox, Roman Catholics, and most Protestants are still
Christians. They still adhere to the Creeds of Nicaea and Constantinople, and also to the theology and Christology embraced at Ephesus (i.e., the anathemas of St. Cyril against Nestorius; that St. Mary is Theotokos). The more fine-grained question is which one of these can plausibly claim to the be
the Church to the exclusion of the others, but that's not usually a question that either interests or includes Protestants, as the idea of strict ecclesiology has very much fallen by the wayside for basically every group other than Catholics and Orthodox. (And it's only more fine-grained because various dematerialized ideas about communion have so long gone unchecked, largely in the Protestant/Catholic/Western Christian world, precisely due to the unpopularity of the idea of strict ecclesiology, so you'll get plenty of people who will agree that Christ is God -- thankfully! -- but then somehow balk at the idea that He established
one Church, not 3, or 20, or 500.)
Long story short: A heretic is usually* still a Christian, though who is a heretic and who is not will depend on who you ask.
(* Important note: some writers of considerable antiquity, such as St. Augustine, use the word heretical/heresy to describe groups and philosophies that never claimed to be Christian and did not originate from within the Christian Church, such as Manichaeism.)