The first programmable, electronic, general-purpose digital computer, completed in 1945. There had been other computers that had these features, but the ENIAC had all of them in one package. It was Turing-complete and able to solve "a large class of numerical problems" through reprogramming.

ENIAC could be programmed to perform complex sequences of operations, including loops, branches, and subroutines. However, instead of the stored-program computers that exist today, ENIAC was just a large collection of arithmetic machines, which originally had programs set up into the machine by a combination of plugboard wiring and three portable function tables (containing 1,200 ten-way switches each). The task of taking a problem and mapping it onto the machine was complex, and usually took weeks.

Although ENIAC was designed and primarily used to calculate artillery firing tables for the United States Army's Ballistic Research Laboratory (which later became a part of the Army Research Laboratory), its first program was a study of the feasibility of the thermonuclear weapon.

By the end of its operation in 1956, ENIAC contained

• 18,000 vacuum tubes
• 7,200 crystal diodes
• 1,500 relays
• 70,000 resistors
• 10,000 capacitors

• ~5,000,000 hand-soldered joints.

  It weighed more than 30 short tons (27 t), was roughly 8 ft × 3 ft × 100 ft (2 m × 1 m × 30 m) in size, occupied 1,800 sq ft (170 m2) and consumed 150 kW of electricity.

This power requirement led to the rumor that whenever the computer was switched on, lights in Philadelphia dimmed.

The basic machine cycle was 200 microseconds (20 cycles of the 100 kHz clock in the cycling unit), or 5,000 cycles per second for operations on the 10-digit numbers. In one of these cycles, ENIAC could write a number to a register, read a number from a register, or add/subtract two numbers.

Several tubes burned out almost every day, leaving ENIAC nonfunctional about half the time. Special high-reliability tubes were not available until 1948. Most of these failures, however, occurred during the warm-up and cool-down periods, when the tube heaters and cathodes were under the most thermal stress. Engineers reduced ENIAC's tube failures to the more acceptable rate of one tube every two days. According to an interview in 1989 with Eckert, "We had a tube fail about every two days and we could locate the problem within 15 minutes."