Valid-invalid bit in a process page table
Typically the page table is a tree of fixed size pages.  So each node of the page table contains a fixed number of entries.  Here's the picture of a 32-bit x86 multi-level page table from Wikipedia:



Each page on x86 is $2^{12}=4096$ bytes. Each page table entry is 4 bytes = 32 bits, and contains a pointer to the beginning of another page.  Each page table page contains exactly $2^{10}=1024$ page table entries.  Since the pages are aligned we know that the bottom 12 bits of the pointer would all be zeros, so we use those bits for status bits (including the invalid bit).  The invalid bit is necessary if you want to make the table sparse in any way.