What does it mean by "terminally blocked" amino acid residue?

Question

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• Energy-based reconstruction of a protein backbone from its alpha-carbon trace by a Monte-Carlo method

For local interaction energy, use is made of a two-dimensional Fourier series expansion of the energies of model terminally blocked amino acid residues, calculated with the Empirical Conformational Energy Program for Peptides (ECEPP/3) force field in the angles lambda(1) and lambda(2) defining the rotation of peptide groups adjacent to a C-alpha carbon atom about the corresponding C-alpha ...C-alpha virtual-bond axes.

What does it mean by terminally blocked amino acid residue?