The mental strength to control one's actions, resist temptation, or persevere.
Dieting requires strong willpower.
He overcame temptation through willpower.
自制心 specifically refers to self-restraint or self-control, whereas 意志力 is broader, encompassing the power to initiate and persist in actions as well as to resist.
Compound of 意志 (will, intention) and 力 (power, strength). The exact historical derivation is uncertain, but the word is a straightforward modern combination.