The meat of a sheep in its first year is lamb; that of a juvenile sheep older than one year is hogget; and the meat of an adult sheep is mutton.
- Lamb — a young sheep under 12 months of age which does not have any permanent incisor teeth in wear
- Hogget — a sheep of either sex having no more than two permanent incisors in wear
- Mutton — a female (ewe) or castrated male (wether) sheep having more than two permanent incisors in wear.
It looks as if there may be different definitions of lamb in different countries, as well. The definition you've found doesn't explicitly differentiate between the terms lamb and hogget, but I imagine the second exists because the meat is more tender than mutton (often considered quite a tough meat), but the sheep is too old to be called a lamb.
So, no, a lamb is not a hogget.