There are many ways to define critical thinking. Here is one way to begin. This is taken from an encyclopedia entry I recently constructed:

Critical thinking is a rich, dynamic, complex concept that entails bringing the most appropriate and highest standards for thought to bear upon the intrinsic (and frequently flawed) reasoning that occurs in the human mind, in order to reason at the highest levels. Critical thinking, in its explicit form, requires disciplining one’s own thinking, as well as understanding and evaluating others’ thinking, by focusing deliberately on the components present in all human reasoning. It requires developing executive-level functioning, in which the mind examines and re-examines its own thought by reasoning about its thinking to improve its reasoning. This entails identifying and examining any errors or problems in the components, or elements, of reasoning, and correcting flaws by employing criteria appropriate in a given context. This systematic process, daily and routinely applied, should increasingly improve one’s reasoning abilities over time. This in turn should lead to progressively more insightful levels of self-awareness on the part of the reasoner and, were it to be taken seriously on a broad scale, on the species itself.

Critical thinking should lead to a world in which the majority of people, through their thinking, are able to make meaningful contributions and find contentment, despite the many complexities we face. Critical thinking arises from the need to intervene in the mistaken, irrational, and self-deceptive reasoning pervasive in human societies, which requires cultivating traits of mind, as well as skills and abilities.

Critical thinking entails getting underneath and examining social norms, conventions, and taboos to determine the extent to which they are logical and reasonable. It might be noted that not every critical thinking theoretician accepts or advances the importance of ethical reasoning in critical thinking. Yet critical thinking as a serious field of study would guide reasoners to embrace and develop within themselves intellectual as well as ethical virtues that gradually improve and enhance their character as fairminded persons; this follows from the ethical obligations all people face in living a human life.

Critical thought differs in logic from any other field in that it attempts to understand thought itself - what thought entails, and where it goes wrong; it attempts this, in part, by uncovering, examining, and appreciating the many ways in which skilled, committed, passionate human thought leads to a more reasonable world, as well as the many ways in which confusions, delusions, and deceptions in human thought lead to a less reasonable world. In its highest manifestations, critical thinking searches for understandings and practices that advance life on the planet. In short, critical thinking is separate from, but necessary to, every subject, discipline, profession, or arena of human life that entails reasoning; this includes virtually every domain of life, to some degree, since reasoning is naturally occurring in the mind.

Critical thinking searches for patterns of intrinsic neurotic and pathological reasoning that impede our capacities for criticality, logical thought, and reasonability. Critical thinking seeks to understand human reasoning as a system of ideas intimately connected with other fundamental functions of the human mind, namely emotions and desires (the affective dimension). Because thinking coexists in relationship with feelings and the human will, critical thinking theoreticians attempt to understand the concomitant relationships among thinking, emotions, and desires, and to uncover the conceptual tools most useful in improving not only cognition, or thinking, but also emotions and desires. Critical thinking is meant to be preeminently practical; it seeks to establish and develop universal concepts and principles about human thought essential to advanced reasoning in any field of study. By improving reasoning, these universal principles are essential as well to improving human life.

In sum, guiding conceptual principles for human reasoning, and hence human life, are explored, established, and cultivated through the field of critical thinking studies.