Crickets are nocturnal insects characterised by long antennae, muscular hind legs for jumping and a chirping sounds the males make. They are related and often confused with grasshoppers. The chirping sounds is specific to every species and created by rubbing particular parts of their hind legs together. The chirp frequency (number of chirps per minutes) is dependent on the temperature of the environment of the cricket as well as on the availability of food or the success in mating.
Amos Dolbear established in 1897 that the number of chirps per minutes of the "Snowy Tree Cricket" (Oecanthus fultoni) can be used to accurately determine the temperature of the environment of the cricket. This formula became known as Dolbear's Law (for the formula see wikipedia).
In The Jiminy Conjecture, Sheldon' knowledge of Dolbear's Law leads to a bet with Howard to determine the species of a cricket they hear in the apartment. Sheldon believes that the cricket is a Snowy Tree Cricket (Oecanthus fultoni) based the chirp frequency and the temperature of the living room, which he knows exactly because of Leonard's and his Roommate Agreement. Howard believes it is a Common Field Cricket (Gryllus assimilis). They hunt and catch the cricket but fail to determine its species with a field guide. In the end they seek help from Entomology Professor Crawley, who identifies the cricket as a field cricket.
Note: It is somehow surprising that Howard and Sheldon failed to determine the species of the cricket once it was caught because the Snowy Tree Cricket is pale green (see picture), while the Common Field Cricket has a brown/black colour (see picture).
Sheldon: I lost this to Wolowitz in an ill-conceived cricket wager.
Penny: What, do they have Wii Cricket now? Well, that can’t be very popular.
Note: She's referring to cricket (sports).