What is open complex in E.coli DNA replication?
The sentence itself is actually the definition of open complex: It is the structure that is created once the DNA double helix is unwinded due to the DnaA proteins.


  This binding promotes destabilization of nearby AT-rich sequences,
  resulting in unwinding of the DNA double helix and the formation of an
  open complex.


I have also found a papera co-authored by Arthur Kornberg himself where the authors self-define "open complex" as well:


  Three tandem repeats of a 13-mer in the AT-rich region are essential
  to the unique replication origin of E. coli and of remotely related
  Enterobacteriaceae. These iterated sequences are identified by
  deletion analysis and sensitivities to endonucleases as the site for
  initial duplex opening by the initiator dnaA protein. This “open
  complex” requires ATP and 38% for optimum formation and stability.


There are several papersb,c that use "open complex" in the same way.

References:

a David Bramhill, Arthur Kornberg, Duplex opening by dnaA protein at novel sequences in initiation of replication at the origin of the E. coli chromosome, Cell, Volume 52, Issue 5, 1988, Pages 743-755, ISSN 0092-8674, http://dx.doi.org/10.1016/0092-8674(88)90412-6.
(http://www.sciencedirect.com/science/article/pii/0092867488904126)

b Ozaki, Shogo, et al. "A common mechanism for the ATP-DnaA-dependent formation of open complexes at the replication origin." Journal of Biological Chemistry 283.13 (2008): 8351-8362. (http://www.jbc.org/content/283/13/8351.full)

c Mukhopadhyay, Gauranga, et al. "Open-complex formation by the host initiator, DnaA, at the origin of P1 plasmid replication." The EMBO journal 12.12 (1993): 4547. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC413885/)