A dactyl is a foot in poetic meter. In quantitative
verse, often used in Greek or Latin, a dactyl
is a long syllable followed by two short syllables,
as determined by syllable weight. In accentual
verse, often used in English, it is a stressed
syllable followed by two unstressed syllables—the
opposite is the anapaest.
The Greek and Latin words δάκτυλος
and dactylus are themselves dactyls. The English
word poetry is also a dactyl. A useful mnemonic
for remembering this long-short-short pattern
is to consider the relative lengths of the
three bones of a human finger: beginning at
the knuckle, it is one long bone followed
by two shorter ones.
An example of dactylic meter is the first
line of Henry Wadsworth Longfellow's poem
Evangeline, which is in dactylic hexameter:
This is the / forest prim- / eval. The / murmuring
/ pines and the / hem locks,
The first five feet of the line are dactyls;
the sixth a trochee.
Stephen Fry quotes Robert Browning's The Lost
Leader as an example of the use of dactylic
metre to great effect, creating verse with
"great rhythmic dash and drive":
Just for a handful of silver he left us
Just for a riband to stick in his coat
The first three feet in both lines are dactyls.
Another example: the opening lines of Whitman's
"Out of the Cradle Endlessly Rocking", his
poem about the birth of his poetic voice:
Out of the cradle, endlessly rocking [a dactyl,
followed by a trochee; then another dactyl
followed by a trochee]
Out of the mockingbird's throat, the musical
shuttle [2 dactyls, then a trochee; then another
dactyl, followed by a trochee]
. . .
The dactyl "out of the..." becomes a pulse
that rides through the entire poem, often
generating the beginning of each new line,
even though the poem as a whole, as is typical
for Whitman, is extremely varied and "free"
in its use of metrical feet.
Dactyls are the metrical foot of Greek elegiac
poetry, which followed a line of dactylic
hexameter with dactylic pentameter.
References
^ Stephen Fry, The ode less travelled: unlocking
the poet within, Gotham, p. 84, ISBN 978-1-59240-248-9 
