Frankly, what to call it is the least of your worries. This is a very technical area of the law and you only have a limited time to respond (11 calendar days before the hearing). I suggest you consult with an attorney immediately! Losing this motion could have big consequences on your case - it may even get the case dismissed, if you are the Plaintiff. It all depends on what the moving party is asking for at summary judgment.
At the very least, review Civil Rule 56. The Civil Rules can be found on the Washington State Court website. In order to defeat a motion for summary judgment, you will need to raise a genuine issue of material fact on the issue presented.
Generally, if you are responding to a motion for summary judgment, it is called a Response, as in "Defendant's Response to Plaintiff's Motion for Summary Judgment" or "Plaintiff's Response to Defendant's Motion for Summary Judgment." I wish you the best of luck and hope you consult with an attorney right away.
I concur with the attorneys who previously submitted answers. I recommend you meet with counsel immediately as the motion for summary judgment could resolve the claims at issue in the case in full in or in part. Your response is critical to showing questions of fact are at issue.