What is resistance? - Definition from WhatIs.com
Part of the Electronics glossary:

Resistance is the opposition that a substance offers to the flow of electric current.  It is represented by the uppercase letter R.  The standard unit of resistance is the ohm, sometimes written out as a word, and sometimes symbolized by the uppercase Greek letter omega: Greek letter omega

When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt exists, then the resistance of that component is one ohm. (For more discussion of the relationship among current, resistance and voltage, see Ohm's law.)

In general, when the applied voltage is held constant, the current in a direct-current (DC) electrical circuit is inversely proportional to the resistance.  If the resistance is doubled, the current is cut in half; if the resistance is halved, the current is doubled.  This rule also holds true for most low-frequency alternating-current (AC) systems, such as household utility circuits. In some AC circuits, especially at high frequencies, the situation is more complex because some components in these systems can store and release energy, as well as dissipating or converting it.

The electrical resistance per unit length, area, or volume of a substance is known as resistivity. Resistivity figures are often specified for copper and aluminum wire, in ohms per kilometer.

Opposition to AC, but not to DC, is a property known as reactance.  In an AC circuit, the resistance and reactance combine vectorially to yield impedance.

Resistance contrasts with conductance, which is a measure of the ease with which electrical current flows through a substance. 

See an introduction to electrical resistance:

This was last updated in July 2015
Posted by: Margaret Rouse

Related Terms


  • geo-fencing (geofencing)

    - Geo-fencing is a feature in a software program that uses the global positioning system (GPS) or radio frequency identification (RFID) to define geographical boundaries. A geofence is a virtual barr... (WhatIs.com)

  • quad-core processor

    - A quad-core processor is a chip with four independent units called cores that read and execute central processing unit (CPU) instructions such as add, move data, and branch. (WhatIs.com)

  • modulation

    - Modulation is the process of adding information to a steady waveform or to an electronic or optical carrier signal, usually applied to electromagnetic signals. (SearchNetworking.com)


  • Electronics

    - Terms related to electronics, including definitions about electrical components and words and phrases about computers, laptops parts, digital cameras, televisions and home appliances.

  • Internet applications

    - This WhatIs.com glossary contains terms related to Internet applications, including definitions about Software as a Service (SaaS) delivery models and words and phrases about web sites, e-commerce ...

Ask a Question. Find an Answer.Powered by ITKnowledgeExchange.com

Ask An IT Question

Get answers from your peers on your most technical challenges

Ask Question

Tech TalkComment



    Contribute to the conversation

    All fields are required. Comments will appear at the bottom of the article.