Definitions of alder:

• noun:   north temperate shrubs or trees having toothed leaves and conelike fruit; bark is used in tanning and dyeing and the rot-resistant wood
• noun:   wood of any of various alder trees; resistant to underwater rot; used for bridges etc
• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #10131)