Dactylic tetrameter is a metre in poetry. It refers to a line consisting of four dactylic feet. "Tetrameter" simply means four poetic feet. Each foot has a stressed syllable followed by two unstressed syllables, the opposite of an anapest, sometimes called antidactylus to reflect this fact.
Example
A dactylic foot is one stressed syllable followed by two unstressed ones:
A dactylic tetrameter would therefore be:
Scanning this using an "x" to represent an unstressed syllable and a "/" to represent a stressed syllable would make a dactylic tetrameter like the following:
The following lines from The Beatles' "Lucy in the Sky with Diamonds" demonstrate this, the scansion being:
Another example, from Browning:
References
Dactylic tetrameter Wikipedia(Text) CC BY-SA