Cable compounds are materials used to prevent the ingress of moisture into telecommunication and power transmission cables. The exact nature of the compound varies depending upon the type of cable in which they are used, but generally they are either soft waxy compounds (for use in copper-wire telecom cables), soft thixotropic gels with a consistency similar to that of hair gel (for use in optical fibre cables), or resin/polymer thickened oils (for use in paper insulated energy cables).
Cable compounds are formulated and manufactured to have very good electrical resistance properties to ensure good functioning of the filled cable.
References
Cable compound Wikipedia(Text) CC BY-SA