One or more electronic devices whose primary purpose is to acquire data. It can be simple or complex, depending upon your needs. Typically a data acquisition system involves at least three main components. First, sensors respond to a physical stimulus and transmit signals or change electrical property such as resistance. Second, a datalogger measures the electrical signal, converts it to a number and stores either that value or some statistics on that value (average, maximum, minimum, standard deviation, etc.). Third, a PC uses some communications link (serial port, phone modem, radio modem, etc.) to retrieve the data from the datalogger. The resulting data is only as good as the sensors can measure and the datalogger can resolve. Reliable communications and software on the PC are critical to retrieving an accurate copy of the data in a timely fashion.

an integrated Analog to Digital converter, internal voltage reference, multiplexer, RAM and (usually volatile) program memory

Any instrument or computer that acquires data from sensors via amplifiers, multiplexers, and any necessary analog to digital converters. Typically associated with process industries.