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.

There is a really nifty chart for this on the pentyl group page of Wikipedia:

$$ \small \begin{array}{lcc} \hline \text{Name} & \text{Structure} & \text{IUPAC status} & \text{IUPAC recommendation} \\ \hline n\text{-pentyl} & \ce{-(CH2)4CH3}\ & \text{} & \text{pentyl} \\ tert\text{-pentyl} & \ce{-C(CH3)2CH2CH3} & \text{Allowed} & \text{1,1-dimethylpropyl} \\ \text{neopentyl} & \ce{-CH2C(CH3)3} & \text{Allowed} & \text{2,2-dimethylpropyl} \\ \text{isopentyl} & \ce{-CH2CH2CH(CH3)2} & \text{Allowed} & \text{3-methylbutyl} \\ sec\text{-pentyl} & \ce{-CH(CH3)CH2CH2CH3} & & \text{1-methylbutyl, pentan-2-yl} \\ 3\text{-pentyl} & \ce{-CH(C2CH3)2} & & \text{1-ethylpropyl, pentan-3-yl} \\ \hline \end{array} $$