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Nachamu is the name given to the Shabbat after Tishah b'Av. The name comes from the haftarah from Isaiah 40 that begins with the word, "Nachamu, nachamu ami", "Comfort, comfort My people". After the three weeks of mourning for the tragedy of the destruction of the Temples that culminate in Tishah b'Av, comfort is very much needed.
Nachamu does not mean only, or precisely "comfort, but its different meanings shed light on what comfort is.
Genesis 6:6 tells that God nichem that he had created humans after they started to corrupt the world. Nichem there appears to mean something like "regret". In Exodus 13:17, the Torah recounts that after the people left Egypt, God did not lead them through the desert by the closest route, in case they would "think a thought" about leaving Egypt, and wish to return. ("Think a thought" is how Rashi explains or word, nichem.)
Commenting on Genesis 6:6, Rashi adduces the other places in the Bible where the root of nechamah occurs, and generalises that all the cases involve machshavah acheret, "having a different thought". This is the common thread linking regret and comfort - thinking differently about a situation. Comfort begins when we can reframe the immediate pain of a loss in a larger, more encompassing picture or story.