English translation: intranet/infranet (see answers below)
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Explanation: I think that the same word would be used in French and in English. "infra" is a prefix that means "under". It is used in words such as "infrastructure" (French: infrastructure) , "infrared" (French: infrarouge), "infrasound" (French : infrason). Obviously, someone has used it with the word "net" (the same as others have used the prefixes "infra" and "inter").
For instance, the university of Waterloo is using it as a name for a project: