Section 155 of the California Corporations Code somewhat circularly defines "board" as "the board of directors of the corporation".   But why does the General Corporation Law and the corporation laws of other states refer to the group or body of directors as a "board"?  

The word "board" is the modern spelling of the Anglo Saxon word "bord" which meant a flat piece of wood.  By the time of Geoffrey Chaucer, the word was used as a synecdoche for a table.  In The Clerk's Tale, for example, Chaucer writes "'Sir Clerk of Oxenford,' our hoste sayde, 'Ye ryde as coy and still as dooth a mayde, Were new spoused, sitting at the bord . . ." ("'Sir Scholar of Oxford,' our Host said, 'You ride as shy and still does a maid, Who is just married, sitting at the wedding table' . . .").  The Canterbury Tales (translated by Peter Tuttle).   See also The Summoner's Tale ("Wheras this lord sat eting at his bord").

Although nothing in the General Corporation Law requires that directors physically meet a table, the term is employed as a metonymy for the assemblage of directors.