Part of a song where the bass player plays, well, a solo. Unlike with other solos, this usually requires that the rest of the band shut up completely so you can actually hear the bass solo. This goes double for jazz or classical rock 'n' roll bands with a double bass or doghouse.
Your typical bass solo consist of figurations around notes, as the bass can't play chords (normally; see below). Enhanced expression is possible with slapping (electric or double bass) and tapping (electric bass only). On a double bass, a good bass player playing fast and hot slaps can make his own groove, as the percussive sound of strings on wood interacts with the bass line. mkb tells me they've seen bowing in bass solos, too.
For nice bass solos, look out on Deep Purple records. There aren't many, but they're good. Metallica has great solo bass work, too. Oh, and of course (thanks, Just Tom) don't forget Flea, bassist extraordinaire from Red Hot Chili Peppers. I've heard someone call him "Hendrix on bass", and while that may be exaggerated, it is true that he does play chords. There's also Les Claypool of Primus who plays a fretless six-string bass, also going beyond the limits of traditional bass playing. (tip o' the hat to myrigth)
Now for the obligatory joke:
A missionary comes into a stereotypical aboriginal village. The sound of drums is in the air. Ceaselessly. Even at night, there is always someone drumming. After one week of continuous drumming, our missionary asks the tribe's chief: "Why do your people keep drumming all day and all night?"
The chief answers: "Many, many years ago, the Great Spirit descended upon our village and told our ancestors that, should we ever stop drumming, the bass solo will set in."
Ta-dah!