Straggler

1. One who straggles, or departs from the direct or proper course, or from the company to which he belongs; one who falls behind the rest; one who rambles without any settled direction.

2. A roving vagabond. Shak.

3. Something that shoots, or spreads out, beyond the rest, or too far; an exuberant growth.

Let thy hand supply the pruning knife,
And crop luxuriant stragglers.

4. Something that stands alone or by itself.

noun

One that lags: dawdler, dilly-dallier, lag, laggard, lagger, lingerer, loiterer, poke, procrastinator, tarrier. Informal slowpoke. See fast/slow/velocity.

n. 1. any personnel, vehicles, ships, or aircraft that, without apparent purpose or assigned mission, become separated from their unit, column, or formation.

2. a ship separated from its convoy by more than 5 nautical miles (through inability to keep up) and unable to rejoin before dark, or separated from its convoy by more than 10 nautical miles whether or not it can rejoin before dark.

See the Introduction, Abbreviations and Pronunciation for further details.

(DOD, NATO) 1. Any personnel, vehicles, ships, or aircraft which, without apparent purpose or assigned mission, become separated from their unit, column, or formation. 2. A ship separated from its convoy by more than 5 nautical miles, through inability to keep up, and unable to rejoin before dark, or over 10 nautical miles from its convoy whether or not it can rejoin before dark. See also romper.

The noun has one meaning:

Meaning #1: someone who strays or falls behind
  Synonym: strayer