What is one-sided?

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Enumeration of Polyominoes - Free, One-sided, and Fixed Polyominoes
... one-sided polyominoes are distinct when none is a translation or rotation of another (pieces that cannot be flipped over) ... with holes (A001419) free without holes (A000104) one-sided (A000988) fixed (A001168) 1 monomino 2 ... domino 3 ... tromino or triomino 4 ... tetromino 5 ... pentomino 6 ... hexomino ...

More definitions of "one-sided":

  • (adj): Involving only one part or side.
    Synonyms: unilateral
  • (adj): Not reversible or capable of having either side out.
    Synonyms: nonreversible
  • (adj): Excessively devoted to one faction.
    Synonyms: biased