tropylium ions

The delocalized carbenium ion, cycloheptatrienylium, C7H7+, derived formally by detachment of one hydride ion from the CH2 group of cyclohepta-1,3,5-triene and substitution derivatives thereof.
Source:
PAC, 1995, 67, 1307 (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1373