A transistor is a semiconductor device used to amplify or switch electrical signals and power. It is one of the basic building blocks of modern electronics. It is composed of semiconductor material, usually with at least three terminals for connection to an electronic circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Some transistors are packaged individually, but many more in miniature form are found embedded in integrated circuits. Because transistors are the key active components in practically all modern electronics, many people consider them one of the 20th century's greatest inventions.

ट्रान्जिस्टर एक अर्धचालक युक्ति है जिसे मुख्यतः प्रवर्धक (Amplifier) तथा इलेक्ट्रॉनिक स्विच के रूप में प्रयोग किया जाता है। कुछ लोग इसे बीसवीं शताब्दी की सबसे महत्वपूर्ण खोज मानते हैं।