IUPAC name of the tert-pentyl group
The retained names “tert-pentyl”, “isopentyl”, and “neopentyl” were still included in the 1993 IUPAC recommendations.
However, according to Subsection P-57.1.4 of the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), these retained names are no longer recommended. The preferred prefix names are as follows:

“tert-pentyl”: 2-methylbutan-2-yl
“isopentyl”: 3-methylbutyl
“neopentyl”: 2,2-dimethylpropyl


P-57.1.4 Retained prefixes no longer recommended
The retained names phenethyl (2-phenylethyl) for $\ce{C6H5-CH2-CH2-{}}$; benzhydryl (diphenylmethyl), for $\ce{(C6H5)2CH-{}}$; isobutyl (2-methylbutyl) for $\ce{(CH3)2CH-CH2-{}}$; sec-butyl (butan-2-yl, 1-methylpropyl) for $\ce{CH3-CH2-CH(CH3)-{}}$; isopentyl (3-methylbutyl) for $\ce{(CH3)2CH-CH2-CH2-{}}$; tert-pentyl (2-methylbutan-2-yl, 1,1-dimethylpropyl) for $\ce{CH3-CH2-C(CH3)2-{}}$; and neopentyl (2,2-dimethylpropyl) for $\ce{(CH3)3C-CH2-{}}$; are no longer recommended; the first name in parentheses is the preferred prefix name.