• 特別とくべつ — special
• 独特どくとく — unique
• 特徴とくちょう — characteristic / feature
• 特急とっきゅう — limited express (train)
• 特価とっか — special price
• 特集とくしゅう — feature article / special edition
• 特許とっきょ — patent

Formed from the radical 牛 (left form: 牜) plus the sound element 寺. The idea is “distinct among cattle” → “set apart” → special. You’ll see トク consistently in compounds like 特別 and 特徴.