A tier 1 vendor is one of the largest and most well known in its field -- often enjoying national or international recognition and acceptance. Vendors may be both manufacturers and value-added resellers (VARs). For example, Cisco Systems Inc. is regarded as a tier 1 vendor in storage area network switching equipment and related technical certifications. Tier 1 vendors may also be a "preferred" vendor that provides products and services to you at appealing prices or other favorable terms.
See also: tier 2 vendor