Temperature can be measured is several different ways:

Oral

Glass, paper, or electronic thermometer
Normal 98.6° F/37° C

Axillary

Glass or electronic thermometer
Normal 97.6° F/36.3° C

Rectal (or "core")

Glass or electronic thermometer
Normal 99.6° F/37.7° C

Aural (in the ear)

Electronic thermometer
Normal 99.6° F/37.7° C

Of these, axillary is the least and rectal is the most accurate. Fever is defined as an oral temperature (or equivalent) of 100.5° F/38.5° C or above.