The influenza virus nomenclature

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Influenza virus was named systematically. For example, A/Taiwan/151/H3N2. What's the meaning of 151?

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151 is the isolate number. There are so many isolates each year. This number indicates that which isolate is this.

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Standard nomenclature

型別/宿主/區域/病毒株號/分離年度 (H#N#)
type/host/geographic source/isolate number/year of isolation (H#N#)

ps. host is often omitted if the host of the virus is human
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There are three genera of the virus, identified by antigenic differences in their nucleoprotein and matrix protein:

A. Influenza A viruses are known to infect humans, other mammals and birds (see also avian influenza).
B. Influenza B viruses are known to infect humans and seals.
C. Influenza C viruses are known to infect humans and pigs.

The A type of influenza virus is the type most likely to cause epidemics and pandemics. This is because the influenza A virus can undergo antigenic shift and present a new, immune target to susceptible people. Populations tend to have more resistance to influenza B and C, because they only undergo antigenic drift, and have more similarity with previous strains.

Influenza A viruses can be further classified, based on the viral surface proteins hemagglutinin (HA or H) and neuraminidase (NA or N) that are essential to the virus' life cycle. Sixteen H subtypes and nine N subtypes have been identified for influenza A virus. Only one H subtype and one N subtype have been identified for influenza B virus. At present, the most common antigenic variants of influenza A virus are H1N1 and H3N2. (Yohannes et al., 2004)

Yet further variation exists; thus, specific influenza strain isolates are identified by a standard nomenclature specifying virus type, geographical location where first isolated, year of isolation, sequential number of isolation, and HA and NA subtype (Yohannes et al 2004). Examples of the nomenclature are A/Moscow/10/99 (H3N2) and B/Hong Kong/330/2001.

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For people whom is interested in influenza naming,
one may check the naming influenza viruses in flu wiki