Gender (性別, Seibetsu, Sex or Gender) is a mechanic introduced in Generation II that determines whether a Pokémon is a male or female in the Pokémon video games. Most Pokémon have even gender ratios. However, some Pokémon have a greater chance of being one gender than another, such as Blastoise which has a 87.5% chance of being male, but only a 12.5% chance of being female. However, there are some moves that will only work on Pokémon with a specific gender.
Some Pokémon have differences depending on whether they are male or female. This can be seen by either physical appearances known as Gender differences or can be seen through some movesets of Pokémon. Two Pokémon, Meowstic and Indeedee, are considered the same species of Pokémon but have different movesets and abilities depending on their gender.
Some Pokémon, while different species, are considered counterparts to each other due to their gender. One example of this is with Volbeat and Illumise who, despite being two different species of Pokémon, share many similar traits such as type, similar stat distribution as well as sharing the same Hidden Ability.
Gender can be an influence for some Pokémon to evolve, as certain Pokémon will only evolve if they are a specific gender or will evolve differently depending on their gender and if certain conditions are met. One case of this can be observed with Combee which will only evolve into Vespiquen if it reaches Level 20 or above while also being female. This differs from male Combee, which can't evolve at all.
There are some Pokémon that have no gender, usually because they are made of lifeless materials like metal and clay, are alien in nature or their gender is simply undetermined. Most Legendary Pokémon do not have genders, with some exceptions such as Latias and Latios which are female and male, respectively.
In Generation II, a female Pokémon of a species with a gender ratio of 7 males to 1 female cannot be Shiny.
In Generation II, both of the Nidorans have their Gender Sign (♂ & ♀) shown twice.
In Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness, the genders of wild Pokémon depend on the number of floor you are on. If the number is odd, any Pokémon that can be male will be male. If the number is even, any Pokémon that can be female will be female.
Before Generation VI, there was a 1 in 3 chance that Azurill can change its gender upon evolving since its ratio was different from Marill (which is 50-50 male or female chance).