Nj or NJ can stand for:

• Nanojoule (nJ), an SI unit of energy equal to 10^-9 joules
• Nj (digraph)
• New Jersey (United States postal abbreviations)
• Napierville Junction Railway (AAR reporting marks NJ)
• Nordjyske Jernbaner, a Danish railway company
• Narva-Jõesuu, Estonia
• Neighbor-joining, a bioinformatic method for the construction of phylogenetic trees.

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