Technical term in electrical engineering for a device that changes voltage.
A transformer is a device that changes voltage.
変圧器 is the formal technical term for a transformer, while トランス is the common abbreviation used in everyday and technical speech alike.
variable transformer
pole-mounted transformer; distribution transformer
isolation transformer
step-down transformer
step-down transformer
Compound of 変 (hen, 'change') + 圧 (atsu, 'pressure/voltage') + 器 (ki, 'device'). The term is a direct calque of English 'transformer'.