Teleconverter

Also called tele extender or tele adaptor.  This is an optical device that fits between a camera body and a lens and extends the folcal length.  Due to close coupling between DSLRs and lenses these need to be made specific for the lens it will be used on.  Some lenses are not suitable for tele convertors.  In other cases a tele convertor can be used with several lenses, in which case the focal length is mutiplied by the same factor, depending on the convertor.  Common ones are 1.4X and 2x.  They should not be confused with extension tubes which look similar but have no optical components.  Teleconvertors often have a concave or diverging lens.

thumbnail

A smaller version of an image used to represent it.  This is common on web pages with a lot of images where the visitor may only wish to see one or two images in detail, maybe even none.  It reduces the download time required to display the page.  The full image is only downloaded if required.  This should not be confused with Icon which is also a small image, but is representational rather than pictorial.

TIFF (Tagged Information File F)

This is a loss-less file format popular in the graphics world for images.  Adobe acquired the copyright, from Aldus Corporation who developed it, when they bought the company in 1994.  wikipedia.

tone mapping

A method of assigning colours, often from a larger bit depth to a smaller bit depth, but maximising the number of tones and hues used.  It is used, but not exclusively, in HDR processing.

transparency

A frame from a positive film.  As this is positive the light areas are transparent and the dark areas opaque and so it can be projected.  This was a popular medium with colour film, but it was also used with black and white.  If a negative is re-photographed on negative film, vou get a positive which would work as a transparency.