The American Academy of Pediatrics book Caring for Your Baby and Young Child indicates that toddlerhood begins with the second year of life, after your child's first birthday: "Your baby enters her second year and becomes a toddler, crawling vigorously, starting to walk, even talking a little." And later includes the ages from 2-3 years old as well saying, "Although your toddler's growth rate will slow between his second and third birthdays, nevertheless he will continue his remarkable physical transformation from a baby to a child." At age 3, this book states that your child is "no longer a toddler" and is considered a preschooler. Likewise, other books about parenting toddlers, including What to Expect the Toddler Years, cover the second and third years of life.
In other publications such as newspapers (especially British papers) or on television news programs, the word toddler is sometimes used to describe a child as old as 4 years. On this site, however, I use the definition shaped by the American Academy of Pediatrics and other parenting authors as a general rule. I always keep in mind, however, that children develop according to their own schedules, so some advice may apply to an older or younger child depending on that child's individual development.