Plato was Socrates' pupil, and most of his best-known work is devoted to recounting the philosophy of his master. Walking the streets of Athens, Socrates dedicated his life to convincing people who thought they were wise that they were in fact ignorant. At the same time, he sought to use this very tactic as a means to encourage people to discover the truth for themselves rather than trusting received wisdom. Socrates pursued these aims through dialogues, in which he asked questions meant to deconstruct apparent truths. Especially in earlier dialogues like the Meno, this process of interrogation (the elenchus) leads only to a state of uncertainty about what originally seemed to be most clear. This is what happens in the Meno, in which the concept of virtue (which people like Meno and Anytus assume to be clear to everyone) is revealed to be a nearly total mystery.
Meno is Socrates' primary companion in the dialogue that bears his name. Meno was a prominent Thessalian visiting Athens at the time of the dialogue (about 402 BCE). He also makes an appearance in Xenophon's Anabasis, where he is described as treacherous and greedy.
The definition of virtue is the central project in the Meno, and it is never completed. The Greek word for virtue is arete, which can refer both to individual virtues like courage or generosity or to the general virtue of a given person. In the Meno, the word is sometimes used interchangeably with "the good"--this is where it gains its most general sense.
Meno's home city-state. At the beginning of the dialogue, Socrates notes Thessaly's recent reputation for wisdom, based largely on the successful careers of Sophists like the famous Gorgias. In light of the dialogue's illustration of the difficulty of knowing what virtue is, this compliment comes to look more like a veiled accusation of pretentiousness.
Gorgias was one of the most well-known of the Sophists, and Plato devotes a later dialogue to him. Here he serves primarily as the foremost example of someone who claims to know what virtue is, and will give expansive, grand answers to questions that Socrates wants to claim are actually extremely difficult. Nonetheless, Gorgias gets off somewhat lighter than the other Sophists, due mainly to his refusal to claim that he can teach virtue (though he would apparently claim to know what it is). Meno is a casual follower of Gorgias.
The Sophists were a class of rhetoricians and philosophers who were widely hired as teachers in ancient Greece. The claims of the Sophists cover a wide range, but they come under fire in the Meno primarily for their claims to understand virtue (and to be able to teach it for a fee). Socrates brings up the Sophists twice in the dialogue, both times to suggest that they are the most likely candidates if one is looking for a teacher of virtue. This suggestion is always somewhat facetious, since Socrates presumably knows what he eventually demonstrates--that no one can teach virtue because no one yet knows the nature of it. Socrates dislikes the Sophists because they give grand, expansive answers embellished with literary references rather than the clear and concise definitions preferred by Socrates. In short, they practice rhetoric rather than true philosophy. The Sophists also came under fire from many prominent members of society (including Anytus, who scorns them in the Meno), who accused them of corrupting the youth by destroying traditional ideals with specious wordplay. Gorgias is the only Sophist named in the Meno (apart from a passing reference to Protagoras), and Socrates accords him less derision than the others due to his relative modesty (Meno says that Gorgias never claims to be able to teach virtue, only rhetoric).
The elenchus is the primary method of Socratic philosophy. Essentially a cross-examination, it proceeds by an intensive series of questions and aims to lead the interviewee to conclude for himself that he or she does not know what he or she thought (a state of uncertainty, or aporia, with regard to the topic at hand). The Meno contains a compact model of the Socratic elenchus in Socrates' examination of Meno's slave on questions of geometry.
In common speech, eidos means "stature" or "appearance." Plato uses it in a much broader sense in his dialogues, where it eventually comes to denote the set of ideal forms of which all worldly things are imperfect examples. These forms can be thought of as abstract models on which things in the world are built. Forms are what all things of a given kind (all chairs, all virtue, all health, etc.) have in common, and the term is used in the Meno primarily in this context. Eventually, Plato will link the idea of forms with the idea of the eternal soul, suggesting that the human soul comes to know the forms in its time out of the human body (see his Phaedrus and Phaedo). Here, however, he seems to be using the term only to denote that which defines all instances of a given thing (namely virtue).
A famous ancient philosopher of the physical world. He is cited in the Meno in the context of Socrates' definition of color, which Socrates bases on Empedocles' concept of effluvia (the elements that enter through our sense organs). The definition is an example for Meno to follow in defining virtue, and Empedocles' name is dropped so that Plato can draw a contrast between the Sophist style of definition (which includes such eminent references) and the simple style of definition encouraged by Socrates.
The state of uncertainty reached when one realizes one is ignorant of what one thought one knew. This is the goal of most early Platonic dialogues, and it is the state in which the Meno closes. Socrates makes a strong argument in the Meno that one is better off knowing one's own ignorance than falsely believing one knows the answer.
Anytus is a prominent Athenian statesman who enters the dialogue toward the end. He has relatively few lines and contributes to the discussion mainly by lending authority to Socrates' point that even the most well-known and upstanding of men are clueless about virtue and the possibility of teaching it. Anytus also makes a short, nasty speech against the Sophists.
Theognis was a poet in the sixth century BCE, and Socrates quotes him to show that even the most gifted and wisest of men are confused about whether virtue can be taught (the lines Socrates quotes contradict each other on this issue).