Television:
A set of scanlines in video to make a complete picture. If the video is interlaced the frame consists of both of the interlaced fields (half frames). If the video is progressive the the frame is made up of one continuous scan from top to bottom. The number of scanlines vary in a frame depending on the TV system used. PAL50 uses 625 scan lines, NTSC60 (US) 525.

Video Encoding:
A frame is one picture but depending on the encoding scheme it may not be a complete picture (I-Frame) but dependent on frames before or after the current frame (P-Frame, B-Frame).