Slickline refers to a single strand wire which is used to run tools into wellbore for several purposes. It is used in the oil and gas industry, but also describes that niche of the industry that involves using a slickline truck or doing a slickline job.
Slickline looks like a long, smooth, unbraided wire, often shiny, silver/chrome in appearance. It comes in varying lengths, according to the depth of wells in the area it is used (it can be ordered to specification) up to 35,000 feet in length. It is used to lower and raise downhole tools used in oil and gas well maintenance to the appropriate depth of the drilled well. In use and appearance it is connected by the drum it is spooled off of in the back of the slickline truck to the wireline sheave a round wheel grooved and sized to accept a specified line and positioned to redirect the line to another sheave that will allow the slickline to enter the wellbore. Slickline is used to lower downhole tools into an oil or gas well to perform a specified maintenance job downhole. Downhole refers to the area in the pipe below surface, the pipe being either the casing cemented in the hole by the drilling rig (which keeps the drilled hole from caving in and pressure from the various oil or gas zones downhole from feeding into one another) or the tubing, a smaller diameter pipe hung inside the casing.