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The N terminal of a protein is the first segment (or residue) in the protein.

The "N" refers to the nitrogen atom in the "amine group" of an amino acid.

Proteins are built in a specific order. The N terminal residue is always one that was added before any others.

"Rockets" cartoon view of protein in Jmol. The arrowheads on the helixes and sheets point toward the C terminal of the protein, and away from the N terminal.

Methionine is always the first amino acid added to a protein, but it's often removed by a post-translational modification (PTM).

The order of segments is important.

Some protein-viewing tools have "rockets", similar to the cartoon view in Foldit, but the arrowheads on sheets and helixes. The arrowheads point toward the end of the protein, the higher-numbered segments.

The last segment of a protein is called the C terminal.