NOUN (2)

1. a body that resembles a spore but is not an endospore; produced by some bacteria;

2. one of a string of thick walled vegetative resting cells formed by some algae and fungi;

The Collaborative International Dictionary of English v.0.48:

Arthrospore \Ar"thro*spore\, n. [Gr. ? joint + E. spore.]
   (Bacteriol.)
   1. A bacterial resting cell, -- formerly considered a spore,
      but now known to occur even in endosporous bacteria.
      [Webster 1913 Suppl.]

   2. one of the small conidia occurring arranged in chains in
      certain fungi. Called also oidium.
      [PJC] -- Ar`thro*spor"ic, Ar*thros"po*rous, a.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

arthrospore
    n 1: a body that resembles a spore but is not an endospore;
         produced by some bacteria
    2: one of a string of thick walled vegetative resting cells
       formed by some algae and fungi