ChemGlobe: Terbium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Terbium: Basic information, atomic structure, and isotopes.
EnvironmentalChemistry.com: Terbium: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Terbium: Provides information on history and properties.
LANL: Terbium: History, production, properties, and uses.
Lenntech: Terbium: Physical data, chemical properties, health and environmental effects.
Terbium: Data tables and historic information.
Visual Elements: Terbium: Image, source, physical information, and key isotopes.
WebElements: Terbium: Extensive information on history, uses, occurrence, compounds, and properties.
Wikipedia: Terbium: Properties of the element, including its history, applications, and characteristics.