The definition can be found in Section 1 of the HTA:
“intersection” means the area embraced within the prolongation or connection of the lateral curb lines or, if none, then of the lateral boundary lines of two or more highways that join one another at an angle, whether or not one highway crosses the other;
Now here's the thing... I hope you're sitting down.
Your insurance claim has nothing to do with the Highway Traffic Act. Fault, for the most part, has nothing to do with the HTA. Instead, it's set by the Fault Determination Rules, which uses a different set of rules and terms. So because you turned left in front of an oncoming car (if I understand the collision circumstances correctly), you'd be assessed as 100% at fault, unless the oncoming driver blew a stop sign or ran a red light. What is relevant for you is section 12 (5) of the Fault Determination Rules, which are made under the Insurance Act of Ontario:
(5) If automobile “B” turns left into the path of automobile “A”, the driver of automobile “A” is not at fault and the driver of automobile “B” is 100 per cent at fault for the incident.
http://www.ibc.ca/en/car_insurance/docu ... -rules.pdf
Sorry to be the bearer of bad news, but it appears as though your insurance company is correct. The dropped ticket will help your rates with respect to "convictions," but not liability for the collision.