Triangle

Basic Shapes used in many symbols
Ashley Davidoff
TheCommonVein.net

 

The triangle is a simple shape but contains a profound meaning which symbolizes strength. The triangle is an ancient symbol of Deity or the divine …

 

https://starbiz.com/love-life/triangle-symbol-meanings-13067

From Symbolism Wiki

In most literary pieces, the triangle (with the number three) represents perfectness, unity, and importance.

A triangle represents the fourth letter of the Greek Alphabet called Delta. It is from the Phoenician Dalet, which is also a triangle. In Physics the Delta is used to represent change.

Maps

In most older maps a triangle is drawn in place of a mountain or mountain range, in modern maps this symbol is occasionally used for mountain in keys or legends.

In Christianity 

The equilateral, having three identical angles, represents the Holy Trinity of the Father, Son and Holy Spirit. This also goes along with Christianity’s extensive symbolism of three. When a circle is added to the center of the equilateral triangle it is considered to represent the eternity with in the Trinity. It is believed that this is the inspiration of the Deathly Hallows Symbol in the Harry Potter series. In the seventh book it is explained that the triangle represents Harry Potter’s Invisibility Cloak

Alchemical symbol for fire
Symbol for en:sulfur, or sulphur, within en:alchemy.
Alchemical symbol for air
Alchemical symbol for earth
The Squared Circle: an Alchemical Symbol (17th century) illustrating the interplay of the four elements of matter symbolising the philosopher’s stone

 

Star of David
2 Triangles
Sign of Biohazard

 

T Valknut symbol involves three interlocking triangles. It is known from the Viking world in two forms. The Borromean form is three interlocking but separate triangles, while the unicursal form is drawn using a single line. It is generally though that these are variations of the same symbol, as archaeologically, they appear in the same contexts.
The name Valknut is not a traditional Viking name, but a modern Norwegian title given to the symbol. It combines the words for slain warrior, “valr”, and knot “knut”, to mean “knot of slain warriors”. This name was chosen as the Valknut appears to be associated with the dead, and with Odin, the god of war and the dead.
A section of the Stora Hammars I stone in Gotland, Sweden depicts a valknut in a central and predominant position, appearing alongside figures interpreted as Odin with a characteristic spear shunting another figure into a burial mound while a raven is overhead and another man is hanged.
The Tängelgårda stone from Gotland, Sweden features valknuts below a depiction of a horse

Links and References

Symbolisms Wiki – The Triangle