definition of a weapon :
weap·on
ˈwepən/Submit
noun
a thing designed or used for inflicting bodily harm or physical damage.
"nuclear weapons"
Some magic creatures have the supernatural ability to instantly heal damage from ***weapons*** or ignore blows altogether as though they were invulnerable.
The numerical part of a creature’s damage reduction (or DR) is the amount of damage the creature ignores from normal attacks. Usually, a certain type of ***weapon*** can overcome this reduction (see Overcoming DR). This information is separated from the damage reduction number by a slash. For example, DR 5/magic means that a creature takes 5 less points of damage from all weapons that are not magic. If a dash follows the slash, then the damage reduction is effective against any attack that does not ignore damage reduction.
Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury poison, a monk’s stunning, and injury-based disease.***Damage Reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact.***
Attacks that deal no damage because of the target’s damage reduction do not disrupt spells.
Spells, spell-like abilities, and energy attacks (even non-magical fire) ignore damage reduction.
Seems fairly clear to me. Weapon definition includes that of a trap, or even a possibly long fall, provided it was intended to harm. Furthermore, DR specifically calls out what it does NOT apply to. Falling damage is not listed.