http://ocw.mit.edu/courses/mathemat...quations-fall-2005/lecture-notes/lecture1.pdf What is [itex] \partial \Omega \text{?}[/tex]

Apparently, [itex]\Omega[/itex] is a volume of space and [itex]\partial \Omega[/itex] denote its boundary. For instance, if [itex]\Omega[/itex] is the unit ball, then [itex]\partial \Omega[/itex] is the unit sphere.