What does Spivak mean by "epistemic violence" in her paper "can the subaltern speak"?

Spivak's term "epistemic violence" means the infliction of harm against subjects though discourse. Spivak's understanding of discourse comes from Foucault.

In the work of Michel Foucault, and that of the social theoreticians he inspired: discourse describes “an entity of sequences, of signs, in that they are enouncements (énoncés)” An enouncement (l’énoncé, “the statement”) is not a unit of semiotic signs, but an abstract construct that allows the signs to assign and communicate specific, repeatable relations to, between, and among objects, subjects, and statements.3 Hence, a discourse is composed of semiotic sequences (relations among signs) between and among objects, subjects, and statements. The term discursive formation conceptually describes the regular communications (written and spoken) that produce such discourses. As a philosopher, Foucault applied the discursive formation in the analyses of large bodies of knowledge, such as political economy and natural history.4

The Wikipedia page on semiotics is very good. I suggest Saussure as background reading to understanding Foucault.

the "father" of modern linguistics, proposed a dualistic notion of signs, relating the signifier as the form of the word or phrase uttered, to the signified as the mental concept. It is important to note that, according to Saussure, the sign is completely arbitrary—i.e., there was no necessary connection between the sign and its meaning. This sets him apart from previous philosophers such as Plato or the Scholastics, who thought that there must be some connection between a signifier and the object it signifies.

According to Spivak, epistemic violence occurred through the marginilization of certain voices within Western discourses. These voices belong to the "subaltern."

It is similar to Said's idea of "otherness" in Orientialism.

Following the ideas of Michel Foucault, Said emphasized the relationship between power and knowledge in scholarly and popular thinking, in particular regarding European views of the Islamic Arab world. Said argued that Orient and Occident worked as oppositional terms, so that the "Orient" was constructed as a negative inversion of Western culture. The work of Gramsci was also important in shaping Edward Said's analysis in this area. In particular, Said can be seen to have been influenced by Gramsci's notion of hegemony in understanding the pervasiveness of Orientalist constructs and representations in Western scholarship and reporting, and their relation to the exercise of power over the "Orient".

The opposite of a subaltern would be hegemony.

Further information, useful for historical scholars:

The subaltern are only people of the former colonial possessions. The current usage of the term will denote this meaning. It does not describe Grasci's understanding of the subaltern, which would describe the proletariat. In this case marginilazation occurs through epistemic violence, although other peoples were historically marginilized through discourse, such as the proletariat, women, slaves, and racial, ethnic or religious minorities. If the process is generalized to any group, the term used is cultural hegemony, as first described by Gramsci. Today attention has been paid to describing the unique histories of these groups.

So marginilazation most clearly describes the process where the history of a group is mostly left out of the writing of history all together. But it is not the only process. These groups were written about and described in the past historical records by the dominant and powerful group. This provided a skewed view about who these people were and how they lived, so emphasis is paid to altering this by including the "voice" of these groups themselves. This is why Spivak asks: Can the subaltern speak? The answer to her rhetorical question was "no."

As referenced before, Spivak's paper and Said's book Orientalism describe this specific problem within historical and other discourses related to the people of former colonial possessions. Spivak says that the West imposes a false universal value system on the subaltern. So instead of accepting the values of the unique culture as valid or internally consistent, the practices will be described as barbaric, strange, immoral, and totally harmful or counterproductive to the cultural and social progress of the people. She uses the British ban on Sati (burning widows alive with their dead husbands) as her example.

Morality is a very disputable subject. I'm going to focus on the internal consistency of culture argument. Foucault realized that cultures could be arranged completely differently than the West but still be somehow internally consistent. He describes this idea in The Order of Things (1966). This passage illustrates the idea, without having to describe the philosophy too much.

This book first arose out of a passage in Borges, out of the laughter that shattered, as I read the passage, all the familiar landmarks of my thought - our thought, the thought that bears the stamp of our age and our geography - breaking up all the ordered surfaces and all the planes with which we are accustomed to tame the wild profusion of existing things, and continuing long afterwards to disturb and threaten with collapse our age-old distinction between the Same and the Other. This passage quotes a ‘certain Chinese encyclopedia’ in which it is written that ‘animals are divided into: (a) belonging to the Emperor, (b) embalmed, (c) tame, (d) sucking pigs, (e) sirens, (f) fabulous, (g) stray dogs, (h) included in the present classification, (i) frenzied, (j) innumerable, (k) drawn with a very fine camelhair brush, (1) et cetera, (m) having just broken the water pitcher, (n) that from a long way off look like flies’. In the wonderment of this taxonomy, the thing we apprehend in one great leap, the thing that, by means of the fable, is demonstrated as the exotic charm of another system of thought, is the limitation of our own, the stark impossibility of thinking that.

The power of discourse is coming from what he calls discursive formations, described in this same book, The Archeology of Knowledge. (Razie is obviously a big fan!) Discursive formations were also discussed on philsophy.SE