A name for the kind of weight-and-pendulum eight-day clock in a tall case, formerly in common use.
Grandfather clock is the colloquial name for the style of clock that is more properly called a Longcase clock. The clocks live up to their name and are very tall - a feature that is alluded to in Henry Clay Work's 1876 song Grandfather's Clock, which is the source of the name:
My grandfather's clock
Was too large for the shelf,
So it stood ninety years on the floor;
It was taller by half
Than the old man himself,
Though it weighed not a pennyweight more.
It was bought on the morn
Of the day that he was born,
And was always his treasure and pride;
But it stopped short
Never to go again,
When the old man died.
Smaller models of longcase clock are sometimes called grandmother (nominally, shorter than 6' 3") and grand-daughter (shorter than 5' 2") clocks. These names derive from grandfather clock and have no other connection to grandmothers or grand-daughters.