Quoted from dear Wikipedia:
The pound or pound-mass (abbreviations: lb, lbm, lbm, ℔) is a unit of mass used in the imperial, United States customary and other systems of measurement. A number of different definitions have been used, the most common today being the international avoirdupois pound which is legally defined as exactly 0.45359237 kilograms, and which is divided into 16 avoirdupois ounces.
The unit is descended from the Roman libra (hence the abbreviation "lb"); the name pound is a Germanic adaptation of the Latin phrase libra pondo, 'a pound by weight'. Usage of the unqualified term pound reflects the historical conflation of mass and weight. This accounts for the modern distinguishing terms pound-mass and pound-force.
The unit is descended from the Roman libra (hence the abbreviation "lb"); the name pound is a Germanic adaptation of the Latin phrase libra pondo, 'a pound by weight'. Usage of the unqualified term pound reflects the historical conflation of mass and weight. This accounts for the modern distinguishing terms pound-mass and pound-force.
