“Uninsured motor vehicle,” for the purposes of N.H. Rev. Stat. § 264:15, shall mean, subject to the terms and conditions of the coverage required therein, an insured motor vehicle for which the liability insurer is unable to make payment with respect to the legal liability of its insured within the limits specified therein because of insolvency. The term shall also include an insured motor vehicle if and to the extent that, at the time of the subject accident, (1) the applicable liability insurance limits are less than the limits of the uninsured motorist coverage applicable to the insured; or (2) the available liability insurance has been reduced by payments to others injured in the subject accident to an amount less than the limits of the uninsured motorist coverage applicable to the insured.