what does -54 mean as an error code

I'm having an issue trying to sync my music from iTunes to my Ipod. Each of these has the current software and it says it cannot sync and gives -54 as a reason. Does anyone know what this means or why my music won't transfer. It's worked in the past, it just stopped all of a sudden and no more of my music will sync. I have over 30gb of available space on my Ipod.