The term christomonism has sometimes been leveled against Karl Barth, John MacArthur, and Dietrich Bonhoeffer. But there doesn't seem to be a canonical definition; most mentions of the term are followed immediately by an attempt by the author to explain it. What is "christomonism" and how is it meant as a criticism?
Christomonism is the heresy of identifying Christ as the singular representation of God. It is a heresy because it denies the Trinity, which has been the traditional foundation of orthodoxy.
Douglas John Hall writes:
Christomonism and the exclusivity that attends it represents, I believe, a failure of trinitarian theology. For a triune understanding of God, the western tradition especially was always tempted to substitute an undialectical monotheism heavily informed by a christology emphasizing the divinity principle and downplaying Jesus' true humanity. The result, in the hands of the simplifiers, is what H. Richard Niebuhr rightly named "a new unitarianism of the second person of the trinity"—or, in the plain and oft-repeated slogan of popular evangelicalism, the simple declaration: "Jesus is God." If all we can say of Jesus and of God is that Jesus is God—all the God of God there is—then we have effectively ruled out all other attempts of the human spirit to glimpse the mystery of the ultimate; and this is all the more conspicuously the case when our understanding of "Jesus," in the first place, is really a dogmatic reduction of his person, his "thou-ness," to the "it-ness" of christological propositions that, most of them, enshrine little more than our own religious bid for authority.
Barth responded to the criticism thusly:
Question: "In what specific way, Professor Barth, does your theology avoid being Christomonistic?"
Answer: "Sound theology cannot be either dualistic or monistic. The Gospel defies all isms,' including dualism and monism. Sound theology can only be 'unionistic,' uniting God and man. Christomonism (that's an awful catchword!) was invented by an old friend of mine whose name I will not mention. Christomonism would mean that Christ alone is real and that all other men are only apparently real. But that would be in contradiction with what the name of Jesus-Christ means, namely, union between God and man. This union between God and man has not been made only in Jesus Christ but in him as our representative for the benefit of all men. Jesus Christ as God's servant is true God and true man, but at the same time also our servant and the servant of all men. Christomonism is excluded by the very meaning and goal of God's and man's union in Jesus Christ."
Personally, I find most examples of supposed christomonism to be off-the-mark. It seems to me that a deep understanding of Jesus as Christ is required to have a deep understanding of the Triune God. Jesus put Himself at the center of God's plan to reconcile with humanity, but also predicted the Holy Spirit will continue the plan when Jesus ascended to heaven. (See John 14.) When I seek out what Barth, MacArthur, and Bonhoeffer actually wrote or said, I find a heavy emphasis on the idea that "Jesus is God", but no denial of the idea that the other persons of the Trinity are also God. It would be remarkable if I did, since the New Testament, and Jesus Himself, had a thoroughly trinitarian theology.
(I want to add additional notes and would rather post a comment than an answer, but the min. 50 reputation is preventing me.)
In trying to figure out the precise origin of the term "Christomonism", the best I can surmise is that it was coined by Paul Althaus (in the German Christomonismus) as a critique of Karl Barth. In the interview quoted by John Ericson's answer, Barth says, "Christomonism (that's an awful catchword!) was invented by an old friend of mine whose name I will not mention." An article points to Paul Althaus as the source of this criticism. Although the article does not state that Althaus coined the term, it may be the case. In short, Althaus accuses Barth of a "monism" in regard to Christ, and he eventually uses "Christomonism" as a shorthand for this charge. The article points to Althaus's 1966 book Die Christliche Wahrheit, but for my theory to be correct Althaus would have to have used the term prior to Barth's 1962 interview.
The term seems to have entered into Catholic theology especially through Yves Congar, who responds to Eastern criticisms of the West as being Christomonistic.